This would have been around 1987 or 88. I tried to convince my father that I read the book so he would sign off on my Book It reading list and I would score a personal pan pizza. He examined the totally uncreased spine and called me out for lying. So, primary reason I don't remember the book is that I didn't even read it.
Here's what I remember: it was number 2 in a series. A party of adventurers are seeking a key, and the overall theme of the series was keys, certainly magical and possibly elemental. This book involved a journey through the desert, and there is an encounter with a djinn (this is where I learned the word djinn, as differentiated from genie). I have some sense that the prior or subsequent book involved water.