It really depends on the branch of astrology:
For a modern take on traditional astrology I like On the Heavenly Spheres by Avelar and Rubeiro, and Kevin Burk's Astrology, A Comprehensive Guide to Classical Interpretation.
On horary astrology I've learned a lot from John Frawley's The Horary Textbook.
For modern psychological astrology (which I do very little of these days), I get the most use out of The Astrologer's Handbook by Sakoian and Acker. Rob Hand's "Planets In . . ." series of books is also useful but they are starting to seem a bit dated to me.