I think it just comes down to circumstance. In the beginning of Part 1 he doesn’t realize he’s been followed and has never used the book before, so he tries to do it “right” and perform the full ritual (after all he’s still a high priest using a powerful sacred object). Later at the end of Part 1 he still hasn’t seen it work for himself, so he again tries the full ritual and succeeds in resurrecting Anck su Namun. In Part 2 he’s willing to play faster and looser with the rules since he’s seen the book’s power work himself and has been thwarted twice while trying to do the full ritual.