Last week in Introduction to Syntax Highlighting I touched on a little bit of VimL. There's an excellent freely available resource for learning more about Vim's scripting language in the form of Learn Vimscript the Hard Way by Steve Losh.
Learn Vimscript the Hard Way is aimed at intermediate Vim users -- you should be familiar with major concepts like buffers, windows, text objects, and pretty much everything in
Each chapter introduces a concept and then includes exercises for practice, inspired by Zed Shaw's Learn Python The Hard Way. The case studies in particular demonstrate not just how to write scripts, but how to think about writing complex Vimscript.
Even if you're not interested in writing Vim plugins, I'd recommend reading Learn Vimscript the Hard Way or at least having a flick through the first edition when it's finished. It'll give you the tools you need to really customise Vim without getting stuck in search engine hell.