The IDA Book, by Chris Eagle

Comprehensive
It describes all major IDA features by starting at the beginning and going all the way to the end. Experienced users may be tempted to skip the first few chapters; resist this temptation and you will discover something new.
Accurate
It is very difficult to be detailed and precise when describing such a complex product. Chris does it excellently well.
Real
Handles real world malware, packers, and obfuscated code.
Profound
This is not just a collection of recipes or tricks, but will give you a better understanding of the IDA architecture, thus saving you from unnecessary frustration. Knowing the limitations of your tool is just as important as knowing its capabilties.