Becoming Functional is a book written by Joshua Backfield that lays out steps to transform oneself into a functional programmer.
It is a short book (under 150 pages) and is a very enjoyable read. I did learn a thing or ten. I refer back to the book whenever I have a suspicion that a certain aspect of my code can be improved.
The most interesting revelation is how my background in hardware design and verification engineers lends well to functional programming. Functional concepts seem to be come naturally to me. I am no expert by any means, but looking at Scala code doesn't frighten me anymore.