Is there a book that explains the memory locations for Atari computers?
There are several books you may find useful. When the original Atari 400 and 800 were introduced in 1979, none of their advanced features were documented and Atari kept the information secret. Shortly afterward, however, Atari changed its policy and several volumes were released containing much detailed information for advanced programmers. These include the Atari 400/800 Hardware Manual, the Atari 400/800 Operating System Manual, the Atari 400/800 Operating System Source Listing, and De Re Atari. These books are heavy reading, but together they reveal almost everything there is to know about the Atari 400 and 800 (most of the information is applicable to the newer XL models as well). The books can still be obtained from some local Atari dealers, user groups, and from Atari itself.
If you're interested mainly in a memory map, the most detailed one is published in Mapping the Atari from COMPUTE! Books. Although this book was written for the Atari 400/800, more than 90 percent of the locations are compatible with the XL computers. Other COMPUTE! Books you might want to investigate are The Atari BASIC Source Book, which contains the complete source code for Atari BASIC; Inside Atari DOS, which contains the complete source code for the Atari Disk Operating System (version 2.0S), COMPUTE!'s Third Book of Atari, which has a 1200XL memory map; and The Atari Collection, which includes a section explaining the most useful memory locations in Atari computers.