Depends on what sort of knowledge you're trying to get. If you just want to be able to point out constellations, I would recommend The Stars by H. A. Rey (yes,

Depends on what sort of knowledge you're trying to get. If you just want to be able to point out constellations, I would recommend The Stars by H. A. Rey (yes, the same guy who wrote Curious George). It's unique in that it displays the constellations in a manner that makes them actually look like what they're trying to represent (which ends up making them much easier to find). There's also some good descriptions of the mythology behind many of the constellations and some other interesting features in the sky that can be picked out with the naked eye or binoculars. There's also a section at the end that discusses the celestial sphere (how the sky changes during the seasons, the ecliptic, precession, etc.). If you want something with more depth, I highly recommend Night Sky from St. Martin's Press. It's a little 300 page field guide that will give you a comprehensive introduction to the heavens and basic astronomy and provides you with enough data to go out observing with. The first quarter of the book is devoted to introducing the fundamentals of astronomy from the celestial sphere, to coordinate systems, to telescope designs, to the magnitude system, to variable stars, and stellar evolution. The rest of the book contains star charts, detailed descriptions of each of the constellations and interesting objects therein, and information on the planets. The only problem is that the book is a bit old, so the tables of data (sunset times, positions of the planets, etc.) are now out of date. Nevertheless, it's well worth buying. If you would like a more serious introduction to astronomy and know some basic physics and calculus, you can do no better than to read Introduction to Modern Astrophysics by Carroll & Ostlie (affectionately known as BOB -- the Big Orange Book). It's about 1500 pages long and covers absolutely everything in astronomy. It's clear, comprehensive, and will likely tell you everything you need to know.