No one has ever written a full-length monograph devoted to the Third Crusade in its entirety. There are of course chapters in the general histories of the crusading movement as well as in the scholarly biographies of Richard the Lionheart and Saladin, but, although plenty has been published on specific aspects, the events of 1187-1192 have never attracted the sort of holistic treatment that in recent years historians have lavished on other crusading expeditions. In The Siege of Acre, John Hosler goes some way towards plugging that gap. His study should also be seen as part of a recent upsurge in publications on the military history of the Latin East. John France's short monograph on the battle of Hattin appeared in 2015 and Steve Tibble's revisionist investigation of crusader armies in 2018. (It is perhaps a pity that Tibble and Hosler's books appeared almost simultaneously--and from the same publisher--with the result that neither was able to benefit from the other.) Michael Fulton's Artillery in the Era of the Crusadesalso appeared in 2018--again too late for Hosler--and, just as I was beginning to prepare this review, my attention was drawn to the announcement of the same author's forthcoming Siege Warfare during the Crusades. 
The Siege of Acre is, as the author claims, the first full length treatment of what turned out to be a military operation of epic proportions. The siege lasted close on twenty-three months (28 August 1189-12 July 1191) and so ranks as one of the longest sieges of the Middle Ages and, in a crusading context, is comparable only to the siege of Antioch on the First Crusade (just over 8 months: October 1097-June 1098) and the siege of Damietta on the Fifth (just over 18 months: May 1218-November 1219). The privations suffered by the crusaders and the high death toll made their perseverance and ultimate success all the more remarkable. The military aspects of the siege have not previously attracted much attention, although, alongside the older works of Rogers, Gillingham, and Lyons and Jackson, mentioned should be made here of a valuable recent article by John Pryor. 
What we have is a detailed narrative of the military engagements for the period in question. The story is well told, and along the way there are some valuable insights. I was particularly struck by the appreciation of Henry of Champagne as a military leader in the period before the arrival of the kings of France and England, and it is reasonable to assume that his proven ability at that time helped make him acceptable as a ruler in Acre following the murder of Conrad of Montferrat in 1192. However, there are problems. Hosler, in common with everyone else who has studied these events, is reliant on the narrative sources produced by the opposing sides. Those in Arabic closest to Saladin are strongly biased in his favour, with only the later Ibn al-Athīr voicing criticisms; those in western languages vary considerably in quality and in proximity to the events. Usually the accounts from the opposing sides mirror each other, but not always. There are problems of conflicting chronology, conflicting figures for combatants and victims, and conflicting understanding of which side came off better in any particular incident. All too often these conflicts cannot readily be resolved. Of course, such problems are not limited to this period: had Hosler chosen to go on to examine Richard's campaign in southern Palestine in 1191-1192, he would have found that the Arabic and Christian sources frequently fail to corroborate each other as to when precisely negotiations between the Christians and Muslims were taking place.
The real problem, however, is that any attempt to construct a detailed narrative from medieval authors who in turn were striving to construct their own narrative is always going to prove difficult. The sources disagree; one source has information that the others lack; the authors themselves are prey on the one hand to mis-information and on the other to their own partisanship. The Anglo-Norman authors--much the strongest on the Christian side--will naturally downplay the contribution of Richard's rivals, Conrad of Montferrat and Philip Augustus. So did Conrad really withhold supplies, as Hosler, following the Anglo-Norman authors, believes, or was it that there were no supplies to withhold? And what is to be made of the William of Tyre Continuations? Both the so-called Lyon Eraclesand the version which is published as the principal text in the Recueil des Historiens des croisades cannot, in the form in which they survive, be any earlier than the 1240s, and so what is to be done with those pieces of information that they, and no other sources, contain? Should such information be repeated, mixed in with other information from near contemporary narratives without a "health warning"? Hosler is aware of these problems, but, despite his protestations (4), he allows himself to produce his narrative juxtaposing information from a range of sources of varying quality. To give just one example: at page 7 he repeats the accusation, to be found solely in the Lyon Eracles, that in 1187 it was Count Joscelin who surrendered Acre; where that story originated is not known, but, as I have recently explained elsewhere, there are grounds for suggesting that this unsubstantiated assertion in a source from sixty or so years after the event it is untrue. 
There are a number of points on which issue might be taken. On the much discussed issue of the execution of the captured garrison, it is odd that Hosler does not consider the idea that Richard gave the order because from his standpoint the prisoners of war were a burden on his resources and he believed that Saladin was deliberately allowing the negotiations to drag on in the expectation that now Acre had fallen the western crusaders would begin to return home. Then again, the comparison with the fall of Acre in 1291 (160-161) seems misplaced: in 1191 Acre was not taken by assault, even if at the moment of surrender such an assault seemed inevitable; in 1291, at the end of an efficiently organized bombardment, the Muslims did storm the city through a breach in the wall and then had to fight their way through street by street.
On a more minor level, Raymond III of Tripoli was not dead when Conrad arrived in Tyre (8) but had gone to Tripoli; the archbishop of Tyre was named Joscius and not Joscelin (9); in 1187 Henry II's money paid for troops at Hattin but not at Jerusalem where the money was used for ransoms (10); I am not aware of evidence for Saladin besieging Tyre from November 1188 until the summer of 1189 (11); did Richard de Templo really go on crusade (18)?; at page 31 it is made to sound as if Asadiyya was a placename.
One final complaint. What this book needs is a decent map of Acre and its environs that would show where the crusaders' fortified encampments were situated, where the Muslim forces that were besieging the besiegers were located, and where those engagements that can be located took place. The diagram at page 137 seems to me to serve little purpose. It is an unexpected omission in a book which, despite my strictures, contains much that is of value.
1. John France, Great Battles: Hattin (Oxford: Oxford University Press, 2015); Steve Tibble, The Crusader Armies, 1099-1187 (New Haven: Yale University Press, 2018); Michael Fulton, Artillery in the Era of the Crusades(Leiden: Brill, 2018); Siege Warfare during the Crusades(South Yorkshire, UK: Pen and Sword, 2019 – forthcoming).
2. R. Rogers, Latin Siege Warfare in the Twelfth Century (Oxford: Oxford University Press, 1992), 212-236; John Gillingham, Richard I (New Haven: Yale University Press, 1999), 155-171; Malcom Cameron Lyons and D.E.P. Jackson, Saladin: The Politics of the Holy War(Cambridge: Cambridge University Press, 1982), 298-330; John H. Pryor, 'A Medieval Siege of Troy: The Fight to the Death at Acre, 1189-1191, or the tears of Ṣalāḥ al-Dīn', in The Medieval Way of War: Studies in Medieval Military History in Honor of Bernard S. Bachrach, ed. Gregory I. Halford (Abingdon: Routledge, 2015), 97-115.
3. Peter Edbury, 'The Lyon Eracles Revisited', in Crusading and Trading between West and East: Studies in Honour of David Jacoby, ed. Sophia Menache, Benjamin Z. Kedar, and Michel Balard (Abingdon: Routledge, 2019), 40-53 at 49-50.