Here is an interesting link from Turtle Rock Studio on how they developed the map for de_inferno. It's very educational for any mapper, in my opinion.
I couldn't decide whether this is the right forum for this post, but it's not a mapping question, and it's not about any custom map in particular. So I posted it here.
** WARNING **
The screens do take forever to load, but patience pays off at the end for the fascinating development timeline.