Useful for inspiration and
The Case of Charles Dexter Ward does emphasise that power comes from complex, long term magical transformations but... the first book someone should read to get a grasp of magic? Erm, no.
I wrote a reply to the initial question about eighteen-months ago recommending
Liber Null which is a good place to start with non-lens specific training. I'd add to that: Crowley's
Magick Without Tears. Crowley wrote it late in his life and much of his need for sensation had dissolved by then. He still uses thirty words where ten would do and his references are dense and unnecessary ("The Law is for All but I'll still mock you for not having a classical education like I did! Hahaha, die peasants!") but it is lucid and a careful reader would come away from it with a good understanding of the deeper mechanics.