OMG IM SO GLAD YOU ASK I HAVE A FEW TIPS:
1) I use Figma/Photoshop. If I use Figma I cut the characters out of their backgrounds using Adobe Express quick actions, and Figma is free! Tho you get limited files (but files can have multiple pages and has an infinite canvas)
2) Since presumably the book doesn’t involve making money off of it, I find free fonts on Behance OR I find a font I really like (Adobe fonts/etc) and then Google it + GitHub or VK to find a free version of it. I DONT RECOMMEND THIS METHOD IF YOU MAKE MONEY OFF THE DESIGNS (copyright issue).
3) I use bright solid colors because they’re more eye catching in search results.