If you learn by reading, books are the best investment you can make, both in terms of time and money. Books are cheap or free. We only have one life, but through books we can experience thousands of lives, learn the lessons of the past, avoid the mistakes others have made for us, and open our minds to new ways of thinking.
I take notes on most non-fiction books I read. Derek Sivers Inspired me to post my book notes online.
Reading someone else’s book notes can give you a general outline of a book, and maybe help you decide whether or not to read it. If you find your interest piqued by a set of my book notes, do yourself a favor and pick up the actual book. The ideas will stick around longer.