A brilliant little book that guides you through all the important (but basic) math that you need to understand as a knitter. This is a no-frills book, written in a simple typeface without a lot of flashy graphic design, but the knowledge is solid and simply explained.
The first few chapters are the kind of information that I'm explaining to people daily working in an indie yarn shop: How to make sure you have enough yarn for a project when substituting yarns, how to estimate how much yarn is in a partial ball, what gauge is and how to measure it and what it tells you, and how pattern repeats in a pattern work.
The second half of the book dives deeper into adjusting a pattern for a different gauge (tldr: don't, find a new pattern) and how to make basic alterations to a variety of different styles of patterns, ranked from easy to challenging to don't bother. The very last chapter is all of the calculations used to make the alterations, and the whole book has excellent references within itself to point you back to the right section for the math.
Overall I thoroughly enjoyed this and would recommend it to any advanced beginner/intermediate knitter looking to further understand their craft. There were a few points that didn't sit quite right with my experience, but I have just have been lucky when I did something.