There was this site that had this book on astral projection you could ether down load the pdf ot buy the hard copy for all I know it may have been this site I remember it was a very large book any one know that name of it or the site ?
Could it be the Tom Campbell trilogy? It's the only "long" book (which could actually come 3 in one) that I can think of at the moment. You cannot download any PDF from the website, but you can read it online. If it's not, good luck in your search!
https://www.my-big-toe.com/explore/my-big-toe-trilogy
No, this was one large book
My Big TOE (Theory Of Everything) was Campbell and 820 pages long, originally published as a single volume, then published in three volumes later.
Robert Bruce's Astral Dynamics was also pretty big: my copy is 547 pages.