De Re Atari ("All About Atari") is a book written by Atari, Inc. employees in 1981 and published by the Atari Program Exchange the year after. Targeted at developers, it documents the advanced features of the Atari 8-bit family of home computers and includes ideas for how to use them in applications. The information in the book was not available in a single, collected source at the time of publication. Atari released official documentation for the hardware and a source listing of the operating system the same year, 1982, but they were not as easily obtainable as De Re Atari.
The book was sold as an unbound, shrink-wrapped set of three-hole punched pages. I do have a copy and have made copies for system shunker Brenski. I will endeavour to make further 'hard' copies available when I get chance (a fairly hefty tome to photostat).
Mapping the Atari, written by Ian Chadwick in 1983, is a location-by-location explanation of the Atari 8-bit family's memory map. It is one of the "must have" references for programming the platform, although it was published somewhat late in the machine's timeline. An updated version covers changes to the operating system and newer machines like the 130XE which followed in 1985.
The book contains not only the memory map, but also includes details of how to use them as well.