I am really enjoying the different perspective! It’s written for teenagers who don’t want to be learning classical Japanese as opposed to (presumably) highly motivated university students. That means the explanations are a lot simpler and some of the less immediately important things in Shirane’s work are left out. At the end of each 助動詞 chapter there is a nifty little overview of the most pertinent information. It helps me determine what is actually important for me as a beginner and what I can come back to later in Shirane’s grammar once I have some experience under my belt. I also enjoy learning about grammar in Japanese because it makes discussing modern Japanese grammar with my tutor easier.
How far along are you with classical Japanese? I have Shirane’s reader as well, so I’ve got that to look forward to, but I’d also like something similar aimed at Japanese high schoolers. There is this series, but the aid offered in those books (modern translation and commentary) focuses on the content more than the language. From what I find on Amazon, there are a lot of teaching-to-the-test grammar books available, but fully annotated classical texts not so much (or I suck at finding them). Right now I am debating buying a guide to one of the 古典 high school textbooks (like this) and/or maybe the textbook itself, but I’d have to have it shipped over, so I am fussing a bit over what particular textbook I should buy… and paper books mean no Yomitan and I am a wuss about looking up vocabulary manually 
I wish I could just walk into a Japanese bookstore and browse 