His work can be usefully classified into three categories: loose essay collections, fiction, and conventionally-structured non-fiction. Prometheus Rising & Quantum Psychology are the closest to conventionally-structured non-fiction (though they both flirt with the loose-essay-collection format) and so, for new readers coming at it from a self-help perspective, they're less off-putting. In particular, Prometheus Rising does a pretty good job of explaining RAW's 8-circuit model (which was also helpful for my emotional self-regulation) in an accessible way -- showing its origins as a doctoral thesis reworked into a textbook.
Of course, almost everybody starts with the Illuminatus! trilogy (and so did I). This isn't necessarily a good idea. There's been at least one casualty from somebody starting with the Illuminatus! trilogy and missing the point (documented in The Cuckoo's Egg), though I guess you can blame cocaine and soviet espionage in that case!