Book of Demons is... interesting, even before you get to its meta element. And four hours later, I just can't decide if it's good-interesting or bad-interesting.
It plays like an ARPG on rails, somewhat literally and metaphorically.
Rail 1: Instead of nice large open expanses (or at least hallways) to traverse, you're stuck in little ruts about as wide as you are, moving forward or backwards (and branching), while the mobs run around freely.
Rail 2: The game organizes itself into gameplay sessions with an estimated length of completion based on your prior play. Having just finished Act 1, I'm looking at times ranging from 10-33min (with one more option longer still locked to me). More importantly, each session increments a progress bar towards endgame. No backtracking. No grinding. You're on a one-way trip down (excluding pitstops in town).
You have a set number of equipment slots, starting at 3, and up towards 10, that have to be purchased in-game. Equipment ranges from passives (reserves mana), spells (consumes mana on activation), items (limited charges, but they can drop or be purchased). Equipment (including items) is effectively a one-time drop, can be upgraded, and can (fairly rarely) be found in enchanted forms. For example, one magical item I pulled gives the normal effect, plus +1 mana.
RNG is
nasty. I started my first game on normal... first piece of equipment I got basically lets me hit two targets at once. Second item gives me a 40% chance of dropping restoratives when I get hit. Third item is a health potion. Over the next hour or so, I get a weapon that has a chance to set the place burning every attack and a piece of equipment that gives temporary immunity to burning. So this is pretty easy... lets try hardcore difficulty: Resistance to slow. Resistance to ice when standing in fire. Chance to slow on attack... which wouldn't be so bad if it didn't cost more mana than I had to equip. Invisibility potion (which I used a lot, constantly running away).
Sigh. (Note: It's not all those items do, but they're kind of awful starter items.)
Leveling lets you pick extra life or mana. Also a freebie restore, and more importantly, particularly for hardcore... once you hit L10 or so, you basically get a free chance to die, once per level. It varies by class, but when the warrior 'dies', you get temporary invulnerability and a knockback.
As far as I can tell-- always pick mana, until you feel like you have enough.
Make sure your walkway is clear-- you don't want to be pincered and unable to run away.
Avoid fighting ranged units head on-- it's easier to dodge arrows when you can move perpendicular to their movement.
The game slows down/pauses(?) when you have your card inventory open, so you can swap stuff in the middle of combat.
You can go back to town, or other previous floors, within a session, as long as you're between floors. That means you can heal up in town/use fountains on previous floors.