It really depends on what you choose to read. When you read a good summary of a book coupled with insightful comments, you may gain better balanced perspectives than reading the whole book itself (which in a lot of cases just present one-sided arguments). Moreover, you might have spent only 1/10 of the time and will likely have a pretty good idea whether it's worth spending more time to read it in full.
A nice example is the Wikipedia article on the book 'Guns, Germs, and Steel' which presents an outline of its theory together with criticism and responses--the latter two cannot be found in the book itself. I read the whole 480-page book a few years ago and I currently remember less than the outline given in that single article. Yes, it was a fairly enjoyable reading experience, but comparing to all other opportunities and hobbies I could be doing, I would have saved the time by reading the Wikipedia article and other summaries & critics instead. Another book I regret reading in full is the 320-page 'Blink' which is well-summarized in a single article: http://en.wikipedia.org/wiki/Blink_%28book%29.
To actually learn rather than having fun with prose and anecdotes, in the amount of time reading one book in full, you can instead explore a summary of ten books in interconnected areas and develop a more complete model of the field. If you pick good summaries and well-cited books (so that we can delve into conclusions without arguing too much about raw data), you can learn a whole lot more and in a more balanced way in the same amount of time. (Unless you are working on a dissertation in that field, too many details are simply unnecessary and could in fact interfere with analysis and understanding--as stated in the book 'Blink' above.)
"I read the whole 480-page book a few years ago and I currently remember less than the outline given in that single article."
It sounds like you're implying that the value of reading a book can be measured by how much of it you can consciously recall at some given instant after you've read it. But what if there's more to it than that? You also mention enjoyment of the experience of reading the book. But is there yet more?
I propose that reading a book can stimulate thought that a mere summary would not. Reading a book can generate ideas that a summary might not. Reading a book can change you in ways that a summary would not. Summaries also frequently omit critical details. Summaries are often biased, unfair, and inaccurate.
You may not remember much about a given book you've read, but that doesn't mean that the most you got from it was whatever enjoyment you had of reading it at the time.
A nice example is the Wikipedia article on the book 'Guns, Germs, and Steel' which presents an outline of its theory together with criticism and responses--the latter two cannot be found in the book itself. I read the whole 480-page book a few years ago and I currently remember less than the outline given in that single article. Yes, it was a fairly enjoyable reading experience, but comparing to all other opportunities and hobbies I could be doing, I would have saved the time by reading the Wikipedia article and other summaries & critics instead. Another book I regret reading in full is the 320-page 'Blink' which is well-summarized in a single article: http://en.wikipedia.org/wiki/Blink_%28book%29.
To actually learn rather than having fun with prose and anecdotes, in the amount of time reading one book in full, you can instead explore a summary of ten books in interconnected areas and develop a more complete model of the field. If you pick good summaries and well-cited books (so that we can delve into conclusions without arguing too much about raw data), you can learn a whole lot more and in a more balanced way in the same amount of time. (Unless you are working on a dissertation in that field, too many details are simply unnecessary and could in fact interfere with analysis and understanding--as stated in the book 'Blink' above.)