Why do people hang onto books? Why do they keep them after they have read them?
When reading books that have valuable lessons, some folks keep the book just in case they missed that one sentence that is life changing or want to read it again. There is enough valuable information. You could reread a really helpful book many times, or read it thoroughly once and reread important parts and give the book away. The time you could spent on reading many times till you got every droplet of knowledge out of that single book gives less then reading other books. Unless you want to be able to recite every single idea from a book, it is unneeded work.
Giving away the book is also liberating and serves other purposes. When you know upfront you will be giving away the book, you are more focused and able to remember the bits important to you. As a bonus, other people will be thankful for giving them the book.