If you can get it for a budget price, then I can recommend the official novel to "Ruins Of Myth rannor", the second "Pools Of Radiance" game. It isn't great, but really nice to read.
If you liked The Hobbit, then ou might perhaps like Thera Wakening, too. It's the official novel to Stonekeep, but it was only bundled with the game. I don't know if you can get it via Ebay or Amazon in used state. It isn't a great novel either, but a bit like The Hobbit in its tone, only much, much shorter and a completely different setting (that of Stonekeep).
If you like "fantasy" in a *very* broad sense, then I'd definitively recommend "Momo" by Michael Ende. And his "Neverending Stiory", too. Both are exceptional books.
Unfortunately "Jim Butto and Luke the engine driver" seems (according to
www.Amazon.com) not available as a reasonable price - the sequel, "Jim Button and the Wild 13" is even more interesting. But both are in fact books for children, although their stories (especially the of the sequel) is unique, imho.
What you also could try to do is read the translated TDE books. There aren't many, but maybe you are understanding Aventuria better after that.
What goes a bit into the horror department are "Skullduggery Pleasant", and the victorian-era dystopia of "Tunnels".
Recently I have read the second "Tinker Farm" book, and I didn't like it at all. Despite it looking like as if it was for children, it is definitively not so - at least not in the last part, which becomes a kind of mixture between a thriller/crime story and horror. Grown-up teenanger might like it, but it is indeed very dark in the end, too dark for my taste.
An entirely different thing against is the official novel to The Dark Crsystal. It showas a few facets not clearly visible in the movie - but in the end it's only a retelling of the movie's content.
The Amazing Maurice And His Educated Rodents is a Discworld book - but it can be read without knowing the Discworld at all. In fact, I heavily recommend it ! - Because it sheds some light on the question of "what is humanity" … A magnificient book, imho. It has won a prize - and that very well deserved, imho. - And don't get yourself fooled by the label "for kids" !
What I'm looking forward to read is his "Long Earth", based on the few things I know about this book.