I recently purchased the PDF of The Tome of Horrors (the Swords and Wizardry version; there also is a Pathfinder version) from Frog God Games.
This thing is huge!
It is 688 pages long (but with 14 pages of advertisements at the end), with a monster description on almost every page (some have two). Some of these creatures are taken from classic AD&D (1st edition) volumes, such as The Fiend Folio and the Monster Manual II.
Along with each monster description is a brief ‘sample lair’ that can be used when designing encounters.
Normally I prefer paper books over PDFs, but in this case the PDF is ideal, as one can print up the relevant pages for a gaming session without having to lug around a massive volume.
I haven’t had time to look at the PDF closely yet, but my initial impression is very favourable!