This volume has its roots a little over fifty years ago in work Kottje began for his Habilitationsschrift. He had intended, as he explains in the preface, to write his thesis on the transmission of Hrabanus's oeuvre. As he started to discover just how many manuscripts there are containing Hrabanus's works, Kottje realized that that project would not be viable, and narrowed his focus instead to the penitentials of Hrabanus and Halitgar of Cambrai. Kottje is perhaps best known for his work on early medieval penitential literature, but has also published widely on medieval canon law and law codes, and on medieval libraries.  He is also one of the foremost authorities on Hrabanus in general, as articles like that on Hrabanus in volume 4 of Die deutsche Literatur des Mittelalters. Verfasserlexikon attest.  Despite the change in direction of his earliest research, Kottje never stopped collecting references to manuscripts containing Hrabanus's works. We owe this release of what were essentially personal research notes to the encouragement of Kottje's colleague and president of the Monumenta Germaniae Historica, Rudolf Schieffer, who offered to publish the handlist in MGH's Hilfsmittel series. Mitarbeiter at MGH helped ready it for publication, especially Thomas A. Ziegler, who is credited on the title page.
Although this is not the large-scale study that the young Kottje had hoped to produce, Kottje offers some brief observations on the transmission of Hrabanus's works in his short introduction. In a nutshell, Hrabanus's works were copied in every century from the ninth through the sixteenth. In Hrabanus's lifetime, and even after his death, transmission was driven in large part by requests for copies of his works, although it isn't clear from Kottje's succinct description if or for how long Fulda or Mainz remained a hub for distribution. Transmitted mostly from monastery to monastery, Hrabanus's works circulated earliest in the Frankish heartlands of Middle, South and Western Europe. Knowledge of his texts spread North and East from the eleventh century on, thanks to the efforts of newer monastic orders, the Premonstratensians, the Austin Friars, and above all the Cistercians, who were especially keen collectors of Hrabanus's works. Those wishing to explore further the role of newer monastic orders in the spread of Hrabanus's oeuvre will find useful Kottje's decision to identify in his handlist wherever possible the order responsible for copying a manuscript.
The second half of his introduction explains how he has organized entries in the manuscript handlist and subsequent list of Hrabanus's works. Kottje also introduces us to his sources, manuscript catalogues for the most part. There follow an abbreviations key and a bibliography of some thirty-five pages.
As one would expect, the manuscript handlist occupies the bulk of the book (1-233). It is organized alphabetically by modern library (Admont to Zwettel), and by shelfmark order where a library possesses more than one manuscript containing a work by Hrabanus. References to Hrabanus's works in medieval booklists are also included. Individual entries vary as to the amount of detail they contain, for, as Kottje makes clear in the introduction, he has gathered information mostly not from his own inspection of the manuscripts, but from catalogues and select secondary literature. At a bare minimum, entries offer a date for the manuscript, provide an abbreviated reference to the catalogue (Kat.) where it can be found, and list the Hrabanus text(s) it contains, usually but not always with folio or page numbers. Where origin or provenance data are known, Kottje gives these in a section labeled Prov. Two further categories of information are provided where applicable: (Lit.) further literature, highly selective; and, finally, remarks (Bem.) drawing the user's attention usually to conflicts in the scholarship over provenance or date.
After the handlist comes a list of Hrabanus's works, organized alphabetically, including at the end Excerpta (labeled thus in manuscript catalogues but otherwise unidentified) and Dubia/ pseudonymous works. Each entry in the list of works provides, in the following order, select secondary literature (Lit.); a reference to printed versions of the text, where available (Druck); a "source" category (Quelle/n) which in all but one case simply provides a reference to Enhuber's notes on the text, and finally the list of manuscripts that contain the text. The secondary literature is highly selective. In the case of Hrabanus's biblical commentaries and related works (e.g. his sermonary for Haistulf of Mainz, and homiliary for the emperor Lothar), it is also economical: Kottje mostly directs the reader to volume 1 of Cantelli Berarducci's Hrabani Mauri Opera Exegetica, Repertorium Fontium, supplementing this occasionally with older bibliography she does not include (Stegmüller references, for example). For some of Hrabanus's other works there are puzzling gaps: no reference to Schipper's work on the manuscript transmission of the De Rerum Naturis, for example.  There are gaps too in Kottje's references to printed editions: no reference to Löfstedt's critical edition of Hrabanus's commentary on Matthew, or Mosetti Casaretto and Rosati's edition of the Cena Cipriani, or Simonetti's recent edition of Hrabanus's commentary on the Book of Judith.  Perhaps Kottje's bibliography reflects editions completed before 2000. I wonder, though, at his decision to exclude from the entry on the Concilia convened by Hrabanus (i.e. the Councils of Mainz in 847 and 852) any reference to the MGH edition of these. One needs to turn to individual manuscript entries in the handlist to find the MGH reference.
These quibbles aside, Kottje makes clear that he offers both the handlist and the list of Hrabanus's works as starting points for future research (XIII). The fact that the majority of Hrabanus's texts are still only available in the Patrologia Latina, and sometimes not even that, is certainly eye-opening, and a call to action for scholars with text-critical inclinations. But my observations above on what is missing in Kottje's bibliography encourage me to suggest another kind of path forward. What Kottje provides here is invaluable, a giant first step toward a more complete understanding of the transmission of Hrabanus's works, but I wonder if paper publication is the best medium for material of this sort. This Hilfsmittel volume presents a lifetime of notes but has the effect of fixing them--setting them in stone, as it were. This is the sort of material that cries out to be continually updated and expanded upon. The future for handlists like this surely lies in the digital realm, in a fully searchable form. This could also, crucially, allow for further contributions as scholars study manuscripts that in many cases do not seem to have attracted a reader since a cataloguer last leafed through them. There would also be space in an online version to list, for example, all the content of a manuscript, not just the texts composed by Hrabanus. Scholars will find this printed handlist enormously useful, but a collaborative, digital project is surely the best way to build on what Kottje offers here.
1. See in particular, Raymund Kottje, Die Bußbücher Halitgars von Cambrai und des Hrabanus Maurus. Ihre Überlieferung und ihre Quellen (Beiträge zur Geschichte und Quellenkunde des Mittelalters 8; Berlin: W. de Gruyter, 1980)
2. Raymund Kottje, "Hrabanus Maurus," Die deutsche Literatur des Mittelalters. Verfasserlexikon vol. 4, 2nd ed. (Berlin: W. de Gruyter, 1983), 166-196.
3. W. Schipper, "Rabanus Maurus, De rerum naturis: A Provisional Checklist of Manuscripts," Manuscripta 33 (1989) 109-118. See also his webpage, where he invites corrections and additions to this checklist: https://www.mun.ca/rabanus/manuscripts.html.
4. Hrabanus Maurus, Expositio in Matthaeum, ed. Bengt Löfstedt (Corpus Christianorum. Continuatio Mediaevalis 174-174A; Turnhout: Brepols, 2000); Rabano Mauro/ Giovanni Immonide, La Cena di Cipriano, ed. and trans. Francesco Mosetti Casaretto and Elio Rosati, Gli Orsatti (Testi dell'altro medioevo 25; Alessandria: Edizioni dell'Orso, 2002; rev. ed. 2004); Rabano Mauro, Commentario al libro di Giuditta, ed. Adele Simonetti (Millennio medievale 73. Testi 19; Florence: SISMEL edizioni del Galluzzo, 2008).