The second book is effectively the Monster Manual.
All the favourites are here. It’s a little bit odd in its organisation. Like Chainmail, the monsters aren’t in alphabetical order – rather they’re kind of in rough groups.
The different colours of dragon are in there along with rules for subduing them.
The treasure section has an extensive part on magic swords – they’re intelligence and ego. (This is something we never really delved into when playing AD&D). Otherwise, again there are all the favourites, like bag of holding.
Note, if you’re reading the 5th printing or earlier, you’ll get references to Ents and Balrogs. These were all excised for the 6th, Original Collector’s Edition print. Ents became Treants, and the Balrog section got replaced with a picture.
Another odd thing, if you’re reading the booklets in order, is the picture of the sea turtle at the back – there’s no particular mention of them as a monster. Instead, sea monsters get their own section in the 3rd booklet.