One of the best overviews of this subject (in my view at least) comes from Martha Crenshaw who, in this article, does a fairly good job of knocking down both Robert Pape (Occupation theory) and Mia Bloom (Outbidding other groups theory).

http://iis-db.stanford.edu/pubs/2194..._Terrorism.pdf

There has been a lot of other work in this area since Pape and Bloom published in 2005. One of the better authors for explaining the motivation/ pyschological side is Anne Speckhard who comes at it from a clinical side and has done some good work on female (notable Chechen) attackers.

(This happens to be my phd area, but I'm more focused on the organisational side than the personal motivation/radicalisation debate).