A general advice regarding an approach to this book
Understanding the book without having looked into the rest of the system seems hopeless to me.
As a more broad possibility and an embedding into the whole history Kants own works and system as well as German Idealism as a whole I would suggest Eckart Förster's The Twenty-Five Years of Philosophy
For the same reason, Paul Guyer's The Cambridge Companion to Kant would be another alternative.
If you, as your comment states, already are an expert in Kant's terminology:
The classical one would be of H. Cassirer, son of Ernst Cassirer, already published in 1938: A Commentary on Kant's Critique of Judgment. Parts of the contemporary review by H.J. Paton (!) are available here.
A more modern work would be the Routledge Philosophy GuideBook to Kant on Judgment
From Robert Wicks, new edition from 2007, which explicitly mentions the lack of literature on the book as a whole since Cassirer's commentary because of the focus on the aesthetics of Kant in the first part of the book, claiming to close the gap in the preface, pages X and XI.
The proposal of @MauroAllegranza John Zammito, The Genesis of Kant's Critique of Judgment (1992) may be worth a look as well, but I don't know about it.