Recently, during my ninjutsu training, I reached a plateau in learning the techniques. The movements during it were correct, but it still felt like something I was doing wasn't quite right.
After reading this I realized what I was doing wrong, as it teaches you not to hold the technique in memory, but to drop it from your conscious mind and let it become a natural movement in the unconscious mind. It's also cleared up a lot of my mistakes, taught me some extra moves and I've learned about the history of the ninja.
This book by Dr. Maasaki Hatsumi (34th Grandmaster of the Togakura ryu school of ninjutsu) is probably the most helpful information I've found outside the dojo. But don't think that you can learn ninjutsu just from reading books; it takes dedication, endurance and intention to learn (in other words, the best training possible is physically training with someone who knows the martial art). For anyone who is looking to learn the history of the ninja or is struggling in their training, I highly recommend this book. For anyone hoping to learn ninjutsu from simply reading books, although this does teach some techniques, it's not as in-depth as other books.