First thought: it helps if you're using what you've learned in some way. Maybe find someone to talk to about what you're learning. (Rubber-duck learning?) Imagine you're teaching someone, and trying to summarize what you've learned.

It might help to read two books on the same topic -- just so things you read in one give you context for the other. (Someone told me they retain information better when it melds with topics they already have some long history with...)