Retention is extremely difficult. I once had a history teacher tell the following anecdote: "We'd brought in a bunch of seniors who'd taken the English History class as freshmen and gave them a test to see which dates they could remember. The only date the vast majority of students could remember only a few years after the class ended was 1066 - the famous date of the battle of Hastings. So, I resolved to fix that. I drilled my students relentlessly on a dozen dates other than 1066: we quizzed them, tested them. And two years later, when we tested this batch of students, they could remember only a single date: 1066."

If you read a book straight through and put it down, you'll be lucky to remember even one two names, a main theme, and perhaps which decade or century it took place in. This is equally true for fiction as non-fiction. Quick, name one character besides Hiro Protagonist from Snow Crash! That's why colleges don't do that. Textbooks use home work problems, closed-book tests, flash cards, and still only manage a minimal level of long term retention.

Techniques like spaced repetition learning[1] are able to achieve marginally better results for disconnected knowledge sets like anatomy or foreign language vocabulary. Memory palace[2] techniques are good for encoding lists accurately with good retention and are very good for sequences. But if you really want to learn something, you have to really understand it, attaching meaning to it from all directions. You need a sufficiently robust network of knowledge that even if you forget one detail, the net can be immediately repaired from the surrounding detail. For example, one of the very few dates in history that I know by heart is January 28, 1986. But I don't remember that date in isolation. I know I was a child when I heard about it, and which grade I was in. I know that it happened in the middle of winter, because the fact that it was freezing in Florida was perhaps the root cause of the O-ring failure. I know Ronald Regan was President because he gave a speech about it. All of which would let me reconstruct most of the date even if I forgot, making it much easier to remember the exact date. You can't remember just one fact - but if are interested enough, motivated enough, you can remember dozens of inter-related and mutually supporting facts about a topic.

One of the things I've noticed about quiz shows like Eggheads or The Chase (which feature expert professional quizzers against relative amateurs) is that even at the highest levels of play there is an enormous amount of deduction going on in what is ostensibly a pure recall task. For example, a contestant may not remember Blaise Pascal's birthday, but they know that his mechanical calculator inspired Leibniz's four operation calculator, and that Leibniz was a contemporary of Newton, and that is enough to place Pascal's birthdate in the 17th century.

One of the main purposes of school is not to give you detailed knowledge, but to give you a scaffolding on which to place later more specific knowledge into context. Without this structure in place, the stuff you pick up during your career would be a series of disconnected facts - and as such, easy to forget. Only the stuff that fits into a larger story, and that gets regularly recalled and used, gets retained. Reading a non-fiction book about a subject sufficiently outside your day-to-day work - like evolutionary biology or cognitive psychology - isn't likely to be retained unless reinforced by further reading.

If you're serious the topic, rather than reading for pleasure, you can take notes, check cross-references, look up opposing points of view, summarize in your own words, make and review a deck of flashcards, re-derive any mathematical theorems, plug example numbers into equations to build intuition, write toy programs to illustrate algorithms, and so on. Students and scholars do these things for a reason, and it's not just because they have plenty of free time! But if you just make a single pass through a popular science non-fiction book... Well, you get out what you put in.

[1]: https://en.wikipedia.org/wiki/Spaced_repetition

[2]: https://en.wikipedia.org/wiki/Method_of_loci