The 50 templates found within will certainly change up your monsters (although I personally find they really work well for NPCs more than monsters), and the few example creatures at the end are nice, but I dislike the focus on statistics over narrative. There's no narrative content contained within this book, where giving little examples of how to use these would've been appreciated not because the DM can't figure it out but because it would break up the content to make it easier to consume.
I would also have loved to see a chart or index, giving the name and page of each template as well as a chart showing the XP change when increasing a CR. The CR adjustments I see in here all seem to be relatively following the DMs Guide but any instance of damage I would have reworded. For example:
Overhead Strike, triggers opportunity attacks from creatures within 5ft who are not prone. Deals an extra xd8 damage. X is equal to 1 + half of the creature’s challenge rating (rounded up). If the target is prone, it deals an additional 2d8 damage.
To me, that 2d8 could be super significant at lower levels and I'd instead suggest something like "this deals an extra die of damage, or two extra dice of damage against prone targets". Although I don't consider that a deal breaker. Oh another recommendation: remove the word "new" from everything. "New Ability" "New Reaction"... templates are designed to add new things, this is just an extra word that isn't needed.
Overall though, totally worth the price.
[4 of 5 Stars!]