This handbook's goal is to teach new learners that verbs are much simpler in Japanese than in most other languages and because of that, you shouldn't be afraid!
It dives into tenses, verb types, and conjugations, providing a solid foundation for verb usage in Japanese. And it reads more like a verb-focused textbook than a typical supplement.
Once you learn basic types of verbs, the book goes straight into teaching conjugations. Each of these conjugations is presented with "sentence patterns" they're commonly used with. Thanks to these patterns, the book covers far more grammar than even some beginner textbooks. Each sentence is in romaji, then Japanese (without furigana), and finally English. At the end of each conjugation section is a practice section where you can use what you've learned.
One thing to remember is most of the content in The Handbook of Japanese Verbs will be taught (or at least, should be) in your textbook of choice. While you shouldn't replace the content there with this book, it can be used as a supplement, especially the practice sentences.
Also, some of the language used is difficult to understand if you aren't familiar with English grammatical terms, but the sentences and practice sections don't require that knowledge.