The SuperMemo site has a lot of good information about spaced repetition and memory: http://www.supermemo.com/
Michael Nielsen has written a thorough piece about long term memory and how he uses Anki. He also provides a condensed version as a series of tweets.
Some medical students have banded together to make a paid-for guide focusing on the use of Anki in medical school: https://courses.ankipalace.com/
Soren Bjornstad has written a series of extensive articles about memory, SRS and how to use Anki effectively.