I second Diggory's recommendation of the official Swift book.
A really easy way to try out the language is to install the no-cost, open source Termux app on any 64-bit Android phone or tablet you have and run pkg install swift to get the Swift toolchain (more instructions here). If you really get into using that, you'll want to get a physical keyboard too, USB or bluetooth, though I regularly spend hours using that app with the touch keyboard.