So one thing I noticed about White Box: Fantastic Medieval Adventure Game is that there aren't that many monsters in it. White Box Zombies has some classic monsters and zombie variations you could add. You can find a few more in White Box Gothic. Still there aren't many monster books for OD&D. Luckily Night Owl Workshop's Beasties has you covered.
I currently have the pdf, though I plan on picking up the print version after Origins. The pdf is 88 pages and those pages are jam packed with material. As the product description says, it has 27 monsters, 6 NPC's, 37 drawings (all done by the primary author Thomas Denmark), 5 maps, and 1 "megadungeon" sample. That's quite a bit for such a small book.
The primary layout is page of monster description and stats followed by artwork for said monster. The stats are simple and presented in OD&D/S&W format. However, there are also conversions for other OSR games, including Denmark's own games like Guardians. The descriptions of the monsters give enough info to make them real, but don't over do it. My favorite monsters are the fetid quasidemon and the Xenoc succubus. The NPC section is really cool and has several unique spellcasters. There are two really cool random charts. The first is 12 Minor Undeads which includes several unique minor beasties. This is followed by 12 ingenious tricks and traps. Near the end of the book we are presented with a flying locust citadel, which is related to the Locust God (a creature mentioned in the text). Finally we have a cool map of the Underworld.
This is a fun monster book. It's definitely worth the cover price and should be a welcome edition to any OSR game. You can grab the pdf and print version at OBS or Lulu.