You say potato, I say... well, you get the point. We all have words in books we pronounce in books. For example, with the word Idris, I say Id-ris while some people say Eye-dris. It all depends, and according to Cassandra Clare the same goes with one very important Shadowhunter tool.
"Okay, the thing is stele is an actual word," Cassie said. "In general it’s used to describe a stone monument with an inscribed surface. When I was trying to come up with the name for the Shadowhunters’ drawing implements, I had a whole list of possible words. I picked stele because of the connection to antiquity, the connection to inscriptions and writing, and because it sounded in my head a bit like “stencil.”"
She also seems to have the same debates with her own friends. "The other thing is that fantasy words and readers have a long history —I had an argument with my friend Christopher Rowe last week about how to pronounce Dunedain. Upshot: neither of us knew. We agreed that the best way to pronounce it is however we always pronounced it in our heads. I am fine with people pronouncing stele any which way. Steely, steel, stellay (what I say.) I think in the film they say stellay. But you should not feel like you need to be restricted by films or audiobooks. You can look it up,or pronounce it how you like, since a stele in the books is not the same thing as a stele in real life."
So, there you have it. "Stellay" is the pronunciation used for stele in the movie and books, but it's really up to the reader to determine how they want to say it.