By the time you’re done solving a real problem with it, book knowledge has become experiential knowledge. And experiential knowledge is something you carry with you forever.
This is the job of a “second brain” — an external, integrated digital repository for the things you learn and the resources from which they come. It is a storage and retrieval system, packaging bits of knowledge into discrete packets that can be forwarded to various points in time to be reviewed, utilized, or deleted.