I have some content on multi-echo combination methods that could perhaps be useful in the book. The first file is a very basic explanation of summation, weighting and averaging; the second is a summary (+equations) of the main combination methods that are used in literature. From a quick read of the book, it doesn't look like this content is explicitly covered. My feeling is that it would fit into the theoretical background section right after signal decay.
See this repo for the content: https://github.com/jsheunis/multi-echo-combination.
Thoughts?