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.

[1] https://hermetic.com/crowley/magick-without-tears/index

[2] https://www.sacred-texts.com/oto/aba/aba1.htm

I'll be sure to go and check that out!

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.

Your description makes me think it's a little gross. Is there something I'm missing as to why such poetry is worth reading?

No, I'm amazed by it's existence at all, I don't find it good - but I'm not a fan of poetry in general.

I think his other writing is more interesting to read, but White Stains is special in how foul it is. The fact it was written at all blows my mind.

Edit: I will add though, Crowley does have a peculiar and breathless way of writing that is noteworthy, but you'll find it in any of his works. And it's neat that he wrote White Stains both in english and in french and they are considered separate works!

Ha - I think you should get back to basics - do what thou wilt is the whole of the law