What are the best guides to start expanding my knowledge of variations? (aside from the forums and website)

I know a good majority, but always love to learn more.

I've seen a few guides and their PDFs available for download, but I'd like to hear (see?) other members' opinions first.

Wolff's COO (country of origin) guide is great, although a bit incomplete.  He also has some other variant stuff on his website.  Including info on some foreign variants.
http://swspaceclub.com/wbobafett/introduction-coos/

Wolff's site is fantastic for the coo and for Spanish variants and the Kenner mold variants. TIG has hands down the best ledy guide around. TIG also has the only glasslite, droids, and soon to be other toy line I can't say guides as far as I know anyway.

http://www.imperialgunneryforum.com/t8072-tig-reference-guides-check-this-thread-for-great-guides-on-ledy-poch-pbp-glasslite-droids-pre-production-fotw-and-more

Thank you both. I'm definitely going to look into it.

Although I'm collecting vintage in general, it always helps to have references for the future (or along the way if I run into a predicament).