Books, as traditionally written, are too big for the amount of information which they typically contain. We need knowledge representation which is capable of coherently and consistently storing information in a more direct, relational format.
If I took a book and stripped it down to the bare information, as you'd like, and then read this new information-pure skeleton, I wouldn't remember it as I would if I just read the book. I might only remember a few facts about Thomas Jefferson after reading a bio about him, but I wouldn't remember any facts if I just read the facts.
My hypothesis as to why this is true (disclaimer: this comes almost entirely from introspection): My mind doesn't store memories like a hard drive, you have to tell it a story (so to speak) to get it to remember what you said. My mind doesn't integrate purely relational data, it integrates narrative data. Yes, an important part of a narrative is context and relational information, but another key part of it is the cadence, the spaces interleaved between the stuff we remember. Unless your point was that we actually need a new architecture for our brains, I strongly disagree, albeit on a purely anecdotal basis.
A longer book is correlated, in the audience mind, with more content, more research done, more of an investment to read it hence more of a payoff ("it's a book you have to study"), and also more time spent reading it so potentially more pleasure, even for non-fiction.
Unfortunately, when you read a lot, these propositions start to become cons, and you'd often rather have just the meat--granted, not in a plastic plate, to run the culinary metaphor, but not with the three-hour ceremonial of the (would-be) fancy restaurant either.
When I think back to books that I've read that felt like they had too much filler, I have to admit that many of the memories are the concepts via the anecdotes.
Quite often I just get partly there by explaining something to myself as if I'm teaching myself, but it seems to have some benefit (but also with the introspection disclaimer there).
But you might find that as other commentators point out, without the original packaging, although the summarized book contains all of the insights of the original, it is hard to retain the information.