Forth is by far the easiest to understand under the hood 'high level' language. It works by defining words from other words and uses a stack to do the bulk of its computation (words operate on the stack by consuming elements and leave their return value(s) on the stack). It's extremely elegant and powerful, I've used it for embedded stuff on very small cpu's (with only a few 100's of bytes of RAM).
Forth has some interesting quirks and has a very long history of success in various fields (space travel, radio astronomy, automotive).
The two books mentioned in this thread are a great intro (Starting Forth and Thinking Forth), for some more history have a look here:
One way to think about Forth is a language that is so extensible that the only way to write your program is to extend it with a DSL to solve your problem.
Forth has some interesting quirks and has a very long history of success in various fields (space travel, radio astronomy, automotive).
The two books mentioned in this thread are a great intro (Starting Forth and Thinking Forth), for some more history have a look here:
http://www.colorforth.com/HOPL.html
One way to think about Forth is a language that is so extensible that the only way to write your program is to extend it with a DSL to solve your problem.