The documentation page https://swift.org/documentation has a link to the most recent ePub version of The Swift Programming Language.
Is there any place to download previous versions of the ePub?
And if not, what’s the best way to request that they be made available? (It doesn’t seem like that belongs in the Swift issue tracker.)
To answer the question "What are you trying to do?”
Since there is now the flexibility in checking Swift version in the code with #if swift(>=3.2), and targets may be mixed between Swift 3 and Swift 4, it would be great to have the documentation for Swift 3.2 (or Swift 3.1) still available.
For versions earlier than Swift 3.1. I think continuing to make available all versions of the ePub that were released under the Creative Commons license, seems to go hand-in-hand with the high degree of transparency elsewhere in the Swift - including being able to look through commits since the inception of the language.