Writing a three-sentence summary immediately after finishing a book, before starting the next one, did more for actual retention than any highlighting system I’d tried before. The constraint of three sentences forces you to identify what actually mattered instead of restating the plot.
I keep these summaries in a single running document, oldest at the bottom, and skimming it once a year works like a highlight reel of everything I’ve read. Books I can’t summarize in three sentences are usually the ones that didn’t leave much of an impression anyway, which is its own useful piece of information.
It takes maybe five minutes per book and costs nothing, which is probably why it’s easy to skip even though it works.