ecl said:
Hey Luke, I've attached a quick sketch of where to cut, as well as some additional info you might need. It's actually quite simple. Before you do anything it might be good to just take a close look at the book, and see what parts are glued where. Basically the text block has end pages, on the front and back, which are glued to the inside covers. If you make a cut as indicated in red (front and back cover), that will sever all connections the text block has to the book cover and will fall right out. Don't cut too deep, or you might cut through your cover, which you don't want. It doesn't take much to cut through the endpages.
On my book I also went ahead and tore off the remaining endcover pages that were left glued on the inside covers, but they won't come off clean though. But it will just be covered up when you put in the new block
If you're looking to eventually replace the text block with another one, you may need to prep the block by reenforcing the spine (usually with some kind of web like cloth) and adding new endpages that will attach to the inside covers.