Although I've been using vim for a long time (almost 20 years), at some point I stopped trying to keep up with the new features. I never really took the time to learn vimscript or go beyond just trying to optimize for my work.
I've recently discovered an ebook Learn Vimscript the Hard Way that has changed that. Rather than being yet another "introduction to vim" book that goes over everything I already know, this book is oriented towards the details of scripting, mappings, syntax highlighting, and other deeper features.
The book is paced well. Each chapter goes over the topic with just enough detail to be interesting and enjoyable. It provides exercises and pointers to good reading spots in the help documentation. This is perfect for me, because without the pointers, vim help is just too overwhelming. (I have never been able to work up to reading the 8000+ lines of eval.txt.)
I'm only half-way through the book, but I have already learned a lot from it. I doubt Steve Losh will ever visit my site, but thank you Steve! (and yes, I did purchase the book too.)