I haven't personally written any books, but I have seen this blog article [1] written by Bob Nystrom. He talks about creating his book (about writing an interpreter) in general, but there is a part dedicated to explaining how he illustrated the book by hand and scanned the pictures to include them in the book.
Maybe such a low-tech solution will suit you.
[1] http://journal.stuffwithstuff.com/2020/04/05/crafting-crafti...