If you want to take the plunge, jhbadger's recommendations of starting FORTH and thinking FORTH are both great. Starting FORTH covers the language itself, whereas thinking FORTH deals more with the philosophy of FORTH and programming in general.

If you really want to get serious, writing your own FORTH, which is a kind of right of passage in the FORTH community, is a good way to learn. While somewhat controversial among FORTH purists (for reasons I don't entirely understand), I would recommend Richard Jones' jonesforth as reference for this: https://github.com/nornagon/jonesforth/blob/master/jonesfort...

Thank you to you and jhbadger for these great links.

There's Leo Brodie's classic books from the 1980s (mostly still applicable today)

https://1scyem2bunjw1ghzsf1cjwwn-wpengine.netdna-ssl.com/wp-...

and its followup Thinking Forth

http://prdownloads.sourceforge.net/thinking-forth/thinking-f...