Book 4 isn't a bad place, but Magick in Theory and Practice is more approachable as a first read. It was written as a series of letters to a female student, and Crowley's Victorian attitudes about the intellect of women made it so it was basically an ELI5.
Magick in Theory and Practice is 'part 3' to the 'Parts 1 & 2' of Book 4, and it is dense and hard to read. I wouldn't recommend MiTaP to most people. The compilation of letters you're thinking of is 'Magick Without Tears'[1].
Personally, I think Book 4, Part 1[2] is one of the best explanations of what Crowley was about. It's essentially an argument for meditation, and will be very familiar to anyone who has read Swami Vivekananda's Raja Yoga. It helps a lot that it was a collaboration between him and Mary d'Este Sturges rather than a solely 'Crowley' writing.
Somewhat unrelated, I'm going through Liber Null, from Pter J Caroll and I find it pretty solid, did you ever read it? It's not Thelema but it's prett close.
