"Freedom is a Two-Edged Sword" may have influenced Crowley in Parsons' favor -- it certainly espouses many of Crowley's own ideas, albeit in a veiled way. Among all crackpot lit, this is certainly one essay worth reading (I can't say the same for S.C.U.M Manifesto, but to each his/her own).
The best biography of Parsons is "Sex and Rockets" by John Carter. The author understands the occult subjects described in the book, and the Hubbard incident fills two whole chapters.
The other biography of Parsons, "Strange Angel" is not as good, especially in the way it sidesteps Hubbard -- the author was obviously quite conscious of the Cult of Scientology's litigiousness and seems to be avoiding a lolsuit whenever L Ron is mentioned.