Many people would say this is a terrible suggestion, I think, but depending on your tastes and style, Peter Johnstone’s 1971 book “Topos Theory” might be good.

…true, it’s exceedingly dry, and has been described as “famously impenetrable”, and I certainly wouldn’t recommend it as an only text to try to learn about toposes from. But I actually found it very helpful when I was first learning Topos Theory — first and foremost because it has really, really excellent exercises, with a big range of subjects and difficulties. Secondarily, I also found that once I’d struggled tooth and nail to understand a construction elsewhere, I could come back to Johnstone and appreciate a really clear, neat, perfectly tuned presentation — albeit one I wouldn’t have been able to get anywhere with on its own.