Wind in the Stone by Andre Norton.
It's not poorly written per se, in fact the writing is downright beautiful in a way. It's very... fluent, for lack of a better word. On the other hand, it's extremely dense and probably the only book I read where I actually struggled to get through because of the language alone. More importantly, it is very, very boring. You think a book in which I had to re-read every other line would leave some impression, but no. Rarely does anything interesting happen, just describing whatever the author feels like. Pointless world building exposition abound. When something does happen it's strangulated by the language. In the end all I was left with the feeling of pride in that I managed to slog my way mess, having already forgotten what had happened and the names of all but the most important characters.
To top off the shitcake of confusion, the synopsis on the back starts from halfway in the book making me feel like a total shit-head through the first half as I kept failing to make connections between the two.