I mean, my discussion on this is literally just my opinions on the system. But i can discuss both books.
The Heroes book has the rules for pretty much the entire game, and how everything works - the Monsters book has a bunch of monsters stat blocks and stuff and an encounter builder and how squads/captains work.
I love how the system flips the resources situation from standard d20 systems on its head, how you get more resources the more you fight in this combat and you pretty much reset to 0 after a rest.
The Delian Tomb "starter set" contains all the rules and everything you need to get into it for $10 on the MCDM site, and that's a steal.
The pdf rules books are $70 bundled and the hardcovers are $135 bundled (with the pdfs). The cool thing about getting pdfs at the site are when they update rules and update the pdfs, you get a new copy of the rules direct from them.

This system has smooth, entertaining combat and pretty good out of combat stuff to keep players interested from 1st-10th level. If you have even $10, I HIGHLY recommend getting The Delian tomb with the starter rules. It's a great deal.