Eric Osborn's book is structured in a peculiar way: each part doesn't stand on its own but it depends on the previous one, and opens the way to the following one--like an overlapping, wavy movement.
The book shows the author's philosophical background and his willingness to put it at the disposal of his readers in this work, in which he provides an historical and linguistic analysis of Irenaeus. For this reason Osborn is very careful of the use Irenaeus makes of the lexicon (cf. for example, pp. 74 ff.), putting it in relationship with other technical jargon (architectural, etc.). In fact Irenaeus creates a new theological language.
Osborn pays close attention to Irenaeus' twofold cultural backgrounds, the biblical-Christian one, and the Greek-philosophical one. Osborn's tone is not always homogeneous: he alternates parts in which he shows Irenaeus' original thoughts with others proposing the thinkers who influenced Irenaeus, with quotations in brackets. Osborn makes a good use of the work of other modern authors.
Very interesting are the comments about Irenaeus' optimistic vision and his joy (cf. pp. 108 ff.). These may be influenced by Osborn's Anglo-Saxon religious milieu. I think that the same cultural root influenced Osborn's interpretation of the question of 'Peter's primacy' and of the 'See of the bishop of Rome' (pp. 129-130). Osborn's position on the above matters remains unclear; nonetheless, the idea that the Irenaeic affirmation of the 'Roman Primacy' is only a cultural prejudice sounds rather captious.
The account given on the Sacraments (pp. 133 ff.) and heresy (pp. 150 ff.) is very interesting, even if it seems that heresiologists that followed Irenaeus hold that the idea that there has been a link between Simon Magus and the other heretics starts with Irenaeus. On the contrary, Osborn thinks that Irenaeus' thought there wasn't continuous regarding the succession of heresies.
It's not clear Osborn's position about Gnosticism. He writes about this in many parts of the book but he doesn't give enough reference to what Irenaeus had to say about it. The author seems to recognize this fault and adds an appendix to his book on this matter. In this last part Osborn (like some other heresiologists) tends to diminish the Gnostic schools considering these as merely philosophical schools. I consider Osborn's opinion too simplistic (pp. 266-267). Gnostic schools have a richer and more complex religious life, community life and ritual practices for them to be considered as mere philosophical schools. The distinction made between the Valentine's thought and the Valentinians, based on the debatable and debated thesis of Markschies [[1]] (cf. the critical essay of Simonetti) [[2]], doesn't take into consideration that we don't have Valentine's original works; so we have to rebuild his thought on secondary elements.
Notes:
[[1]] Ch. Markschies, Valentinus Gnosticus? Untersuchungen zur valentinianischen Gnosis mit einem Kommentar zu den Fragmenten Valentins, Tubingen 1992; Ch. Markschies, "Valentinian Gnosticism: toward the Anatomy of a School," in J.D. Turner, A. McGuire The Nag Hammadi Library after Fifty Years. Proceedings of the 1995 Society of Biblical Literature Commemoration, NHMS XLIV, Leiden-New York-Koln, 1997, pp. 401-438.
[[2]] M. Simonetti, Valentinus gnosticus. Due note bibliografiche. Cassiodorus 1, 1995, pp. 197-205.