There is so much great content in this book so I'm just going to name a few of the things I enjoyed in it. If I went through everything interesting or cool we'd be here all day.
My favorite monster has to be the glacier hound. It is like taking a hell hound and turning it on its head. Very neat. Although honorable mention goes to Liche Lord for its similarities to a Lich in such a way that it can make players mistake it for one and pee their pants.
Favorite Sidekick is hands down Vrondrac Maul, the Hobgoblin Thief. Hobgoblins are quite large and I picture him more as someone who brains someone with a club and steals their coin purse instead of sneakily pickpocketing it. So he's like a thug. I actually looked it up and for 5E it says "Hobgoblins are between 5 and 6 feet tall and weigh between 150 and 200 pounds." I could have sworn I read in previous editions that hobgoblins were quite large, over 6'0 at least.
Lord Subane is a rather interesting diety: "Lord Subane is a being of madness, of mind-blasting truths, of things man was not meant to know. To worship him is to risk his attention, which is something to be avoided." -- Very Cthulhu-esque.
The Ring of Time Stop is super powerful. I would kill to have one of these as a player as you can be incredibly imaginative with it. I played a time stopper/traveler in a GURPS supers game. The things you can do with Time Stop are limited only by your imagination.
Deep Rest is a great spell since you quite quickly -- even when starting a game at level 1 -- become unable to put anything to sleep with the 1st level sleep spell due to monsters having more and more HitDice as the campaign progresses. Instead of Sleeps low HD total, Deep Rest works just like Sleep but gives 5d8 HD put to sleep and is a 2nd level spell.
All in all, this was a fun supplement to read. I'll likely use some of the content in the game I'm running right now. Definitely worth a read. I highly suggest checking it out if you play 5E.