I've tried collecting some but didn't have much luck.
Digital Foundations :: https://www.flossmanuals.net/digital-foundations/
Practical introduction to digital art using free software (Inkscape, GIMP, Scribus, etc.).
GIMP Tutorials :: http://www.gimp.org/tutorials/
Tutorials on the official site. It couldn't really find any free books or something comprehensive for GIMP, it seems like people prefer making tutorials that teach tricks.
Grokking the GIMP :: http://gimp-savvy.com/BOOK/
Free book about GIMP, but a bit dated.
Inkscape Manual :: https://www.flossmanuals.net/inkscape/
Introduction to Inkscape, a free (as in freedom) vector graphics editor.
Anime Vectoring Tutorial :: https://sites.google.com/site/jjaaba/main
Quick introduction to vector tracing images using Inkscape. Not limited to anime!
Inkscape tutorials :: https://inkscape.org/en/learn/tutorials/
Tutorials on the official site.
Inkscape Screencasts :: http://screencasters.heathenx.org/
Video demonstrations of some cool Inkscape tricks.
Scribus HOWTOs :: http://wiki.scribus.net/canvas/Category:HOWTO
Scribus is a free and open source desktop publishing program. You can make beautiful PDFs using it.
Scribus Video Tutorials :: http://wiki.scribus.net/canvas/Scribus_Video_Tutorials
Collection of various video tutorials.