Wow, pardon me for saying, but this is one of the most unusual questions I have seen here. I hope that your desire to feel sad is not due to some mental condition. If it is I suggest seeking help (believe me there are lots of avenues out there for that kind of thing) instead of trying to wallow in the sadness. Of course, all of this is just conjecture on my part, so if you simply want to read a book about this topic in order to feel sad because you want a nostalgic experience then you can ignore my (unasked!) advice.

There are plenty of self-help books that might deal with this topic, but I have read a few ficiton ones that might be what you are looking for. One such novel that springs to mind is Never Let Me Go by Kazuo Ishiguro. It is a heartbreaking book about a woman who works for a carer for organ donors. During her youth she attended a boarding school in England where all the students were essentially clones that were only kept alive for organ donations. I'm not going to say much more about the story as I'm already giving away too much, but it definitely made me sad and there's no denying that for the kids at that school their youth was wasted for the gain of others.

Thirteen Reasons Why by Jay Asher is another good one if you want to feel sad because it deals with the suicide of a teen and the tapes that she left behind to explain her reasons for killing herself. The book deals with a lot of the topics that the youth deals with these days, such as bullying, harassement and sexual assualt, and I can think of anything more wasteful than the life of a bright young girl being snuffed out because of these issues. I've even heard of some schools banning this book because there were a bunch of suicides that occured at these schools, but I don't know if the book had any involvement in them.

The last one I'm recommending is Girl in Pieces by Kathleen Glasgow. It's about a young girl named Charlotte who at seventeen years old has already experienced more hardship and loss than many other people has had in their entire lives. Charlotte is treated at a menteal health facility because she has a habit of cutting herself, but due to a lack of insurance coverage she also loses this avenue of help. This leaves her all on her own to deal with everything she has lost during her youth. If you really want to feel sad then I can honestly not think of a better book than this one. It had me sobbing like a little girl in places and I'm not someone who is easily moved by books. If you had to deal with loss or hardship in your own youth, then you'll definitely be able to relate to Charlotte and her struggles.