Is there a dictionary dedicated to word prefixes? I'd like to know more about de-, but there's no uniform meaning; for example, in demystify it signifies a reverse action, while in delimit it's a positive meaning, like set.
I don't think there is a dictionary dedicated solely to prefixes. But you can find some common prefixes and their definitions in normal dictionaries. E.g. definitions of the prefix de- may be found in YourDictionary:
Edit: On the second thought, there are some. You can try searching for "Merriam-Webster's a Dictionary of Prefixes, Suffixes, And Combining Forms". I'm not sure if we're allowed to post links to resources, where you can actually download one though, else I'd give you one.