I found Secret of the Soul by William L. Buhlman a good read. It explores more of what astral projection is and experiences while not talking much about technique.
http://www.amazon.com/exec/obidos/tg/detail/-/006251671X/qid=1061981711/sr=8-3/ref=sr_8_3/002-2212805-8936867?v=glance&s=books&n=507846
http://www.amazon.com/exec/obidos/tg/detail/-/006251671X/qid=1061981711/sr=8-3/ref=sr_8_3/002-2212805-8936867?v=glance&s=books&n=507846