Thank you very much!!! I think I can tell the name of the book (and if I can't, I hope nobody else reads this lol!); it's the Spanish translation of a UK book called "Dark Lord: The Teenage Years" [link] It's coming out in March, but only here in Spain I'm afraid.
Mostly because it was commissioned work (book illustrations pretty much) and the publisher won't let me post anything before the books are published. The first one comes out in March so maybe then they'll allow me to post something :3