Chiming in as a newbie who is looking for the same.
First off, before you spend $20 here, $25 there, and another $30 elsewhere for books that you may end up throwing in the trash, I suggest you decide if you'll study MODERN or TRADITIONAL astrology.
Most of the books I've seen tend to be geared toward the MODERN astrology student. I have decided to focus strictly on TRADITIONAL so I can learn Horary. Fortunately (and unfortunately) this has limited my available reading material.
That said, my first astrology book (checked out from a local library) was "KISS Guide to Astrology" by Julia and Derek Parker. (MODERN) This book was a good overview for me because two months ago I couldn't match the 12 Zodiac symbols with their signs, and I'd never heard of an Ascendant or Rising Sign. Still, I'm glad I checked it out, but even more happy that I didn't pay money for it because I don't see it as a "keeper" book.
I found a 1947 copy of Nicolas DeVore's Astrology Encyclopedia (which I have referred to many, many times as I read posts on this site). Tonight, I located a copy online:
http://www.scribd.com/doc/2578598/Encyclopedia-of-Astrology-Nicholas-deVore
I've currently got two texts going, and I switch back and forth:
"Christian Astrology" by William Lilly (TRADITIONAL) Despite the title, it has nothing to do with religion. You can download all three "books" online here:
http://www.skyscript.co.uk/texts.html I think most, if not all, TRADITIONAL astrologers will say this is a "must read", I should tell you that, for me, it's a brain bruiser that is difficult to absorb. It's 800 pages (a lot of printer ink and paper!) and was written in the 1600s in a somewhat archaic style... which is why I also have a second book going at the same time.
The second book is "Astrology: Understanding the Birth Chart" by Kevin Burk. (He seems to be a TRADITIONAL astrologist, but he's also included the outer planets and comments for MODERN astrology students.) I'm not sure how I feel about his chart interpretations. I've only followed him on one of the 4 or 5 in the book. But I have to say that Burk's book has helped me better understand aspects and planet dignities. I think it's a good beginner reference book just for that reason. Non-beginners will likely consider it a waste of money.
As someone keen on learning TRADITIONAL astrology, I've found BobZemco's detailed and (and often caustic) posts invaluable. Someone else might be able to recommend a different poster on this site if you wish to learn MODERN. While much of what Bob says goes over my head, I find the more I read, the more I begin to understand.
For my next book, I'm considering either "Horary Astrology Plain & Simple" by Anthony Louis; "Martial Art of Horary Astrology" by J. Lee Lehman; or "Raphael's Horary Astrology" by Raphael. (I'd love input on these three if anyone would like to comment.)