I am working on a book about my history with e-sports, nerd culture and being a commentator for matches.
In the final chapter of the book I talk about my friend (who's been helping me sort over a decade of my diary to write the book) in cosplay attending an event with me.
What I was wondering if there be copyright issues if I had a drawn version of that moment for the back cover of the book?
Personality rights of the depicted persons
If that artwork contains a recognizable likeness of your friend (or anyone else), you need their (written!) permission. You are using their likeness as part of the marketing of your book, and they have a right to decide whether and how their likeness is used. Find an example for a "model release" (e.g. one for photographers that work for stock image agencies such as Getty Images) and use it as an example.
Copyright of the artist
If you commission that artwork specifically for that book, make sure the artist gives you (written!) permission to use it for that purpose (and every other purpose that you might want to use it, e.g. on social media, your website, in ads, in other upcoming books etc.). Sample contracts for that can be found online. If you expect a lot of revenue from the book sales, you might want to consult a lawyer to help you properly phrase that contract (as well as the one where your friend allows you to use their likeness).
Trademark or copyright of the represented character
If the image shows you and your friend or anyone else dressed up as a copyrighted or trademarked character, you must get permission from the rights holder of that character. Using such an image inside a book for the purpose of documentation in the context of science or education is likely unproblematic (see fair use), but the use on the back cover serves marketing purposes and requires permission by the rights holder.
