Rivet Your Readers several times, because I keep giving it away to fellow writers. It's really short--less than 100 tiny pages--but it's inexpensive, and I think it's one of the most helpful books writers can have.
Point of View (POV), in my opinion, can make or break writing. When we as authors understand deep POV, it can dramatically improve the flow of writing, create more engaging characters, help with active vs. passive voice, and help with the age-old show vs. tell.
When we as readers are immersed in deep POV, we feel and experience the story along with the character, rather than being a detached observer. Nelson calls it creating "immediacy and intimacy" with readers, and I've seen a huge difference in my own writing and that of others after working to develop this skill.
To be a good writer, an understanding of perception, person (first, second, and third), and tense are essential. This book clearly and concisely explains those things, then gives simple before and after examples to demonstrate.
I don't claim to be an expert on POV, so this little book may not be as in-depth as some would like, but it's absolutely a good place to start for newbies and a good review for the rest of us. It's a book that I would recommend owning. It's a great reference tool, and it never hurts to have a refresher!