Kant’s Life and Thought 
by Ernst Cassirer (Yale University Press, 1983)
A classic that originally appeared—in German—in 1918.

Kant’s Critique of Pure Reason: An Introduction
by Jill Vance Buroker, AM’68, PhD’74 (Cambridge University Press, 2006)
A primer to Kant by a fellow UChicago philosophy graduate.

Hegel’s Critique of Liberalism: Rights in Context
by Steven B. Smith (University of Chicago Press, 1991)
For those interested especially in Hegel’s political philosophy.

Hegel on History
by Joseph McCarney (Routledge, 2000)
Focused on Hegel’s philosophy of history, it offers a very clear, brief introduction to Hegel’s overall system.

German Philosophy: A Very Short Introduction
by Andrew Bowie (Oxford University Press, 2010)