I'm not exactly sure but this looks like this is made by a company called Advameg, Inc. They've quite a few other sites that might be of interest to HN readers:
Would be interested in knowing if someone has any insights.
0 - http://www.city-data.com/neighborhood/Battery-Park-City-New-...
1 - http://www.city-data.com/school/edward-r-murrow-high-school-...
I'm not certain I get how they can afford so much (seemingly?) original material, yet be low-key enough that I haven't come across them before.
1. secure the rights to the book (he knew the author personally/through family)
2. cut the book open and run it through a high resolution scanner
3. use imagemagick to preprocess images
4. run OCR on the pages and convert them to markdown
5. have a compiler convert his markdown and images to HTML
EDIT: FOUND THE LINK: https://news.ycombinator.com/item?id=4974055