I'm not going to provide a full decription of everything since some things are self-explanatory or you'd better discover yourself
what's going on.

This mod allows you to gain and use an extra resource - Daedric Faith to purchase followers, spells, items and fuel special powers.

Here are some details :

1. Progression

You gain faith points for :
1) Trapping souls with Daedric Soul trap ( available in 3 variations ). Works on most creatures and humans but doesn't work with undead and daedra themselves ( 5-8 pts ). Still works on saints and seducers - you sacrifice them to Sheogorath. Probably it's gonna be your main source of income regarding this faith economy.
2) Doing daedric quests (+50, the biggest faith reward per action )
3) Reading some books. Each instance of the same book grants faith.
4) Sacrificing daedra hearts (+5-6 pts)
5) Being a daedric prince ( with a power once a day , +20-25pts)
You can gain more faith from souls if you curse yourself with Daedric Devotion. The curse is revertable but very hard to cheese.

2. Purchase :
Each purchase requires faith and total faith. Total faith is the amount of faith you've gathered while just faith is expendable. Most of the purchases are quite expensive so your playthrough will basically turn into soul hunting, doing Daedra Princes' bidding and other things like these. There is no way to obtain all the new content fast. When you see the menu, the number in brackets tells the faith that you need. The total faith needed is revealed in the message in case you fail to have enough faith to purchase an item ( check with quicksave-quickload)

1. Minions with the Book of Daedra ( not a vanilla book with the same name ). To get the soultrap and spells for your minions such as teleport or on-self group buffs, request a scamp once you read it. They are mortal but there's no limit to how many of them you can recruit. Usually you won't need more than 2 at a time, since even having 2 clannfears will make it hard to enter a dungeon.

Available minions are : scamp, daedroth, clannfear, spider daedra, hunger ( absorbs health instead of fatigue), dremora, dremora mage (spell up to master) and xivilai.

2. Powers ( Book of Forbidden knowledge) - basically spells that require faith to work so that they spend faith per target or cast. These include :
Fire/Frost Sacrifice - explode yourself with the 100 points of damage of the respective element on 100 feet. Needs 1pts to work and journeyman destruction

Summon Sigil Stone - summons a sigil stone for free for 60, but to target it you need 1pts. Casts fireballs dealing magic damage for 30 seconds per target and if you activate it heals 25 pts. It doesn't follow you and doesn't move at all. It's basically an npc that can't be attacked ( there was no other way to make it fly and heal you on activation ).

Meteor - summons a meteor dealing 250 fire damage in 150ft area, but quire inaccurate especially if you move. Also adds 50 percent fire shield for 10 seconds, so that it's good for dark elves. Requires 3pts of faith and expert destruction.

Telekinetic Bomb - sends everyone flying in 30ft area and requires 3pts of faith and journeyman mysticism

Daedric Swarm - spawns 3 leveled daedra nearby. They are hostile unless you are a daedra yourself. Requires 3 points of faith and journeyman conjuration.

Join the kyn - read in the Dremora section below.
3. Mostly regular spells in the Book Of Spells:

Lava blast : fireball that absorbs health. Its instant effect does it as a ranged health absorption while duration based effect absobs it like health absorbtion. Requires journeyman destruction.

Dream : restores 50hp for 5 seconds but gives 250 magic & normal weapons vulnerability for 5 seconds. Cast for free and is a mysticism spell of journeyman level.

Trecherous spikes : 150 reflect damage but drains 2000 fatigue for 10 seconds thus paralyzing you. Requires journeyman mysticism

Bound Staff : summons a staff that deals 75 points of elemental damage. Requires Expert destruction.

Mirror: 100 spell reflection for 5 seconds, also journeyman mysticism.

Coldharbor wind : cold damage that only absorbs fatigue. Cheaper than damage fatigue. Expert level allowing to paralyze enemies.

4. New Daedric artifacts in the book of artifacts. Most of them are very expensive and I encourage you to discover it yourself. Some of them have new skins.

5. Unique spells in the book of secrets:

Curse Flesh - journeyman destruction, reduces armor of the enemy ( to negative values allowing to deal more damage)
with weapons )

Tear Magic Connections - expert mysticism , ignores reflect/absorb on enemies and gives them -50 spell absorbtion. May be useful against Xivilai or liches.

Subjugate soul - expert mysticism, deals health damage bust is resisted by willpower ( Drain Soul ) . Also contains 5 damage willpower for zero extra cost and a daedric soultrap in 10ft area. Very effective against monsters but innefective against spellcasters

Chanel magic force - remote magicka absorbtion in a 10ft area. Costs some magicka itself. Doesn't absorb if you have full magicka.

Also the book allows to request magicka and health permanently ( 2000 total faith to start )

6. Powerful utility spells in the book of devotion :

Spinner's web - frenzy ( up to level 100 ) + invisibility ( expert level )

Dream Snap - freezes enemies in place , but they can receive damage. Cannot be absrobed or reflected , expert mysticism, requires 3 points of faith per target

Mark a Target - -125 magic & physical damage resistance on target for 25 seconds. Expert Destruction.

Axe of Malacath - summons an enchanted axe with high damage and are based damage health enchantment.

Daedric devotion - makes you 100 vulnerable to magic, but you receive twice as much faith from souls. Choose again to remove the curse, it's free.

3. Unique extra effects, which you may encounter from spells obtained from artifacts :

1. Mora's Snap - destruction, damages health, resisted by spell absorption in %.

2. Blade mail - shield + reflect damage

3. Mystic Shield - spell absorption + reflect damage

4. Summon Librarian - summons a very strong dark elytra with a paralysis spell.

4. Dremora

You can play as a dremora now. Dremoras receive faith differently, for example they receive more faith from souls but less from books and sacrificing daedra hearts. Also "Join the kyn" in the book of Forbidden Knowledge allows to join their faction, so daedric swarm will be more useful to you since you are a full daedra. Doing the main quest is gonna be weird. Do not choose join the kyn option if you don't want to become a full dremora. Choosing it several times while gaining total faith allows you to progress in the daedra faction from churl to markynaz. I would add some quests to it, but I don't want to make it bloated. Dremoras and daedra are now immune to lava damage.

5. Locations

The books look like Misterium Xarses and are located in book shops ( including the most essential one ) , somewhere in the mages guild, in an apparently daedric dungeon and a seemingly abandoned house.

Bugs and incompatibilies :

1. Sometimes followers may not follow you through the doors. Try teleporting them or reentering the doors.

2. If you encounter a bug with a quest book, let me know. It might happen, but I didn't face any.

3. Should be compatible with most mods. If the mood changes book scripts, some ways to obtain faith may be unavailable. Your main source of faith is souls anyways.
MOO - actually tested with it without a single ctd. Bear in mind, that spawnable minions are weaker than their enhanced counterparts in this mod. However, the sigil stone may go unconscious once the summon spell effect is over, which makes it harder to use it for healing.