Ray-Casting Tutorial for Game Development and Other Purposes (1996)
permadi.com
permadi.com
Also check out this other nice raycasting tutorial: http://lodev.org/cgtutor/raycasting.html
I recently browsed flipcode.com and sadly parts of it are broken and only available thanks to the Internet Archive.
Those are just for understanding, to go deeper, into code, perhaps the most important thing I found was here:
http://rainmak3r.altervista.org/voxel/ukvoxel.htm
if anything is broken I believe archive.org has copies of the zip files as well, not just the html.
Just wanted to stick it here so perhaps someone else will find it easier to find, it took me a lot of googling and wandering through abandoned forums, looking for out-of-print books that no one has bothered to digitize, etc.
Thank you F. Permadi for the good times!