Seconding the Natively recommendation; you should definitely be able to find other books at a similar level to its L23. (A lot of what Natively has at that kind of level is manga, but not exclusively so.) Or going the other way if you find something you’d like to try reading you can see if Natively has a level for it to see if it would be a step up or not. The Narnia books haven’t yet had enough gradings by readers to reliably peg them at a level, but I suspect they’ll end up being a bit more difficult than L23.
People vary on whether they want to read translated novels – personally I have mostly avoided them because I feel more enthusiasm about reading something I haven’t before and where perhaps I wouldn’t have the option to read it in English; but I know others find it helpful to be effectively rereading a story they’re already familiar with so they don’t get too lost. I do think that it’s important to find something you want to read, whatever that might be, rather than feeling obliged to pick the “getting started” books that are often recommended, because getting started with reading in a foreign language is hard work and if the book isn’t something you’re having fun with it’ll just be an awful slog.
魔女の宅急便 | L23 (the book that was the source material for the Kiki’s Delivery Service anime) comes in at L23, and the WaniKani beginner’s book club read it a few years back, so the discussion threads for their read-through are still on the forum. I remember reading and enjoying that.
Or you could see what the beginner’s book club is reading now (more often manga than books):