Steve Losh has written a book called Learn Vimscript the Hard Way.
It’s badly titled, imho. His definition of the book explains why I say
that: a book for users of the Vim editor who want to learn how to
customize Vim. With that description in mind, I think the book
achieves its goal — a goal that all vimmers would aspire to master.
However with the title of the book, I fear even many proficient
vimmers would assume that the material is out of their reach or too
dense to absorb right now with their busy schedules, relegating it to
a later reading pile, at best.
For all you up and coming Vimmers looking to read something to take
you beyond all of the beginner tutorials out there, read chapters:
0-18, 28-32, 43-48, 50 & 56.
For those of you who picked the book up specifically because of its
title, that review is coming soon. :-)