Despite it's age (1986, I haven't seen the newer edition and don't know how it may have been changed) this is still the best general discussion of nanotech I have seen. Broderick's "The Spike" from 2001 is more generally singularitarian but with a lot of discussion of nanotech and is more readable than Drexler. More technical works are Drexler's "Nanosystems" from the 1990s and "Kinematic Self-Replicating Machines" which is online here - http://www.molecularassembler.com/KSRM.htm