The holy grail of Bonaventure scholarship is John Quinn's Historical Constitution of Bonaventure's Philosophy. This is a huge book, nearly a thousand pages long, and best obtained from a library.
A much respected older work is Etienne Gilson's The Philosophy of St. Bonaventure. A word of caution: Gilson likes to read (or misread) history according to his 'Existential' Thomist view-point.