Seems like five years ago, you couldn't talk about editing a graphic without somebody raising their nose into the air, listing all the expensive software you just had to have to do it right. Luckily, people are starting to come around to the idea that there shouldn't be one software provider to rule them all, and I can't think of any software that exemplifies this struggle better than GIMP (Gnu Image Manipulation Program), the humble graphics editor that started back in 1996 and has held its ground against elitism, software bloat, and platform-bias for over 20 years.
That's important to lots of us, but especially to us gamers. If you create content, whether it's maps, character portraits, book covers, player handouts, pawns, or any number of game-related material, GIMP offers a cheap (usually $0, depending on how you obtain it) and open source way to create.
GIMP is on version 2.10 at the time of this writing, so this book is technically outdated now, but it's still one of my favourite books about GIMP and indeed about photo and graphic editing. Its subtitle is From Novice to Pro, and it takes that seriously. If you've never edited a photo beyond applying basic styles on your mobile or desktop photo app, you can pick this book up and learn not just GIMP, but the finer points of making graphics on a computer. And if you've done graphic editing, you can still learn cool tricks, as well as the intricacies of advanced GIMP usage (including automation).
That's one of the best qualities of a technical book: as it ages and falls out of date, you can ignore the technical parts and still learn from everything else. Better yet, author Akkana Peck set up a website at gimpbook.com, which isn't maintained by any means, but it links to lots of great resources, examples, and tips.
If you want to get familiar with graphic editing, this book is a great resource to have on your shelf.