Thomist Realism and the Critique of Knowledge by Etienne Gilson

 Gilson's penetrating analysis of the difference between realist and critical philosophies. 

Socrates meets Kant by Peter Kreeft

This is an excellent entry in the "Socrates meets ..." series from Kreeft. A good introduction to Kant.

Kant's Theory of Knowledge by Justus Hartnack

This is an excellent, succinct introduction to the Critique of Pure Reason. Hartnack is outstanding at getting across the meaning of Kant's technical terms.

Kant's Life and Thought by Ernst Cassirer

Good take on Kant.