My first book, Beginning Data Science, had a decent length at a bit more than a hundred thousand words.
After that, the books just got shorter and shorter, with Functional Programming at 31K words, Object-oriented Programming at 28K words Metaprogramming at about 24K words (Metaprogramming was published before OOP, but I wrote OOP before Metaprogramming).
With Functional Data Structures I’m reversing the trend. This book won’t be as long as the data science book, but it might end just a bit short of 50K words, so it will have a decent length for a technical book.
It is still a bit short for a text book, but for covering a single technical topic, I think it is an appropriate length.