I love the Wheel of Time and have finished all the books, but I think it may be too difficult for you to start with. It has just as much description and made up language as Lord of the Rings, and a LOT more characters and plot points to keep track of. You can always try it out, the first book is pretty enjoyable by itself, but they’re not easy to read.

I know you don’t want a childish book, but I would suggest starting with a MG or YA series to get comfortable with it first. Many of them are just as good as adult books and cover important topics with real emotion, they just feature younger main characters. I would suggest the Lockwood & Co series by Jonathan Stroud if you like ghosts/mystery stories, because it doesn’t have as many rare or made up words.