Amazon.com has an author’s page for Hastumi-soke here. All books authored by Hatsumi are great reads, but the two that are considered the “timeless classics” are Ninjutsu: History and Tradition and Essence of Ninjutsu.
Soke’s writing style is very interesting, and you can see how he “plays” with various kanji in order to bring a new outlook on the subject. If you read one of his books and wait a month, you may see something different within the same set words. If you read one of his books and wait a year, you will see something different within the same set of words. Having your interpretations change over time is also a good indicator of whether or not you’re advancing in your training.
Training is not just a set of physical skills. There’s a mindset as well, and Soke’s books will help develop that.