When I looked at this book in first edition printing back in 1989, I thought it was a terribly skimpy selection of Crowley's writings. I was also concerned that the heavy editorial content interwoven with the selections would skew a natural, spontaneous reading of Crowley's work. At that time I felt that reading Crowley in situ instead of in anthologies was a much better approach. Now I have come around to see "Portable Darkness" not so much as a selection of Crowley's work, but as an extended critical essay on Crowley with sufficient original source material (Crowley's own words) thrown in to make an informed judgment on whether you like Crowley, and on whether the editor/commentator is making good points. As such it is completely without precedent and invaluable. The author is an extremely intelligent man.
I should also add that the material on so-called western tantra is very complete in itself and includes some works simply not available elsewhere, unless you are a member of an occult group. For this reason alone, it is an invaluable addition to any occultist's library, particularly occultists with a respect for Crowley.
If you want to round out your Crowley library, I highly recommend "Book 4, Magick in Theory and Practice," in the very usefully annotated edition prepared by the OTO head Hymanaeus Beta; "Magick without Tears," less profound that Book 4, but easier to read; "Gems from the Equinox" which purportedly (but does not quite) includes all the magickal writings from Crowley's original opus "The Equinox;" and "Holy Books of Thelema" which includes all the "revealed" or transmitted, Class A writings.
Some of (maybe most of) Crowley's writings are as impenetrable as Blake, Wordsworth, Shelley, and Keats, which is to say, not impossible but certainly poetically grandiose and mind-numbing for anyone but a hardcore English lit major. Book 4 or Magick in Theory and Practice is an exception, the only true grimoire (grammar or rules of magick) produced in the 20th century (Bardon straddling the line between a grimoire and the ultimate self-improvement book for the aspiring occultist).
Finally, the summer beach book par excellence is Magick in Theory and Practice in the inexpensive, non-annotated Dover edition. While you won't be able read the Greek or Latin, unlike Beta's "Book 4" you will be able to carry it in a backpack without getting a hernia.