I'm already rereading the most valuable books and it does help to retain their knowledge.
I guess doing spaced repetition every x time would help even more.
I feel that rereading and to a larger extent spaced repetition are very wasteful.
I think spaced repetition is not every constant set of time but rather every increasing set of time, like reviewing in a week, then a month, then a year, etc. basically reviewing when you're about to forget. I don't think it's even necessary to review it in the same manner, as long as it causes you to remember, it's probably fine.
As examples of reviewing in different manners, you can "review" by applying that knowledge or learning on top of it. Like you can read about an alphabet, then review by practice writing it, or review by learning words that use that alphabet. In that way, "reviewing" is not a waste, as you can gain new knowledge or experience. Spaced repetition is merely trying to improve retention by putting increasing spacing between the recollections of that knowledge.