Cullen's Bonaventure is exhaustive ... and exhausting.
The series Great Medieval Thinkers perhaps sets up its authors for frustration. The series introduction states that the books are designed to be accessible to the general reader with no prior knowledge, while still engaging and entertaining (?) specialists. Most of the books in the series cut toward the general reader. This one may confound the specialists.
Cullen's book can be seen as a counterpoint to the trend of presenting a mystical, cosmic, theological, developing, almost Eastern Bonaventure, read largely through his later works (à la Ewert Cousins)and interpreted through grids (such as the coincidence of opposites) not explicitly found in Bonaventure's works. Instead, Cullen lays out a locus by locus exposition of Bonaventure's thought, following the order of sciences laid out in his De reductione and the theological topics found in his Commentary on the Sentences.
The chapters are information overload. Terse formulas are jammed next to each other with insufficient exposition, just the opposite of a good introduction. Because so many details are treated, the book has a flat quality. The relative predominance of concepts is hard to judge. The reader gets no sense of Bonaventure as a synthetic thinker.
On the other hand, the comprehensiveness of the book makes it useful as a reference tool. It does genuinely capture the breadth of Bonaventure's thought; far from a specialist, he wrote on every theological topic of his day. Bonaventure the philosopher shines through here in a way that he does not in some other introductions. Also, Cullen helpfully distinguishes Bonaventure from Thomas, a useful feature for those who tend to equate scholasticism or even medieval theology with Thomism.