I am interested in learning how to do DFT+U calculations and was wondering if anyone has any good recommendations for books or articles.
I recommend this as a go-to source now. We have discussed DFT+U quite a bit on this very site: GO.
Most modern books that cover DFT now cover DFT+U as well, as I believe to be the case in David Singh's book.
Really though, you absolutely need to check out Kevin J.M.'s answer, it is extremely helpful, organized and gives the most relevant resrouces (Cococcioni, Marzari and other giants' works).
Hopefully this gets the ball rolling!