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Level 79 skills
These each require 20 Apocalypse Pages and 10 medium ink. Apocalypse pages drop in TT2-2, TT1-3, and FB89 from the mobs. You can also exchange them for tokens - 18 Tokens of Perfect Luck gives you 5 Apocalypse pages. The books can be made in Old Heaven's Tear and in 1k streams.
Each of the 89-99 skills come in demon and sage variants.
Level 89 skills (FB89)
Of the level 89 skills available to you, 3 will drop in either Brimstone Pit (sage FB89) or Eden (demon FB89). These are the only level 89 skills that are relatively easy to get. On the average, each boss will drop a book 50% of the time.
- Demonic Vexation
- Demon - Stone Rain
- Demon - Garrotte
- Demon - Plume Shot
- Sage - Tiger Maw
- Sage - Ironwood Scarab
- Sage - Frost Arrow
- Demon - Stomp of Beast King
- Demon - Purehearted Blessing
- Demon - Frost Arrow
- Sage - Venomous Scarab
- Sage - Stream Strike
- Sage - Stone Rain
- Ethereal Abomination
- Demon - True Form
- Demon - Tiger Maw
- Demon - Blazing Arrow
- Sage - Great Cyclone
- Sage - Fox Form
- Sage - Gush
- Demon - Great Cyclone
- Demon - Ironwood Scarab
- Demon - Gush
- Sage - Garrotte
- Sage - Blazing Arrow
- Sage - Draw Blood
- Demon - Fox Form
- Demon - Stream Strike
- Demon - Pyrogram
- Sage - Purehearted Blessing
- Sage - Stomp of Beast King
- Sage - Take Aim
- Thousand Year Spirit
- Demon - Draw Blood
- Demon - Venomous Scarab
- Demon - Take Aim
- Sage - Pyrogram
- Sage - True Form
- Sage - Plume Shot
Level 89 skills (fb99)
All other level 89 books that once dropped from the last boss in either fb99 (either Hellfire Abomination or Peachblossom Ritualist), are now dropped in Flowsilver Palace by the last 3 bosses (Century Golden Toad, Tyrant Prince Mushi, Blossom Succubus).
Level 92 skills
The way to get these is to craft the books from 20 Old Book Pages. These come from the bosses in God's Rebirth Delta (level 86+ requirement). The bosses start dropping these at stage 4.
An Old Book Page can also be obtained by forging 10 Book Credit Cards in the cube. Book Credit Cards are found as rewards from several of the 51-60 rooms, starting at level 96.
Level 92 skills and 99 skill books through Advanced Mystical Pages
You can also acquire 30 Advanced Mystical Pages and get a random skill book through the Mystical Librarian in City of Thousand Streams, or if you collect enough Advanced Mystical Pages, you can get a set of skill books. One Advanced Mystical Page can be created using 15 Perfect - Tokens of Luck.
You will need the follow amounts depending on your class:
Psychic - 270 Advanced Mystical Pages
Assassin - 300 Advanced Mystical Pages
Archer - 270 Advanced Mystical Pages
Cleric - 300 Advanced Mystical Pages
Barbarian - 330 Advanced Mystical Pages
Venomancer - 360 Advanced Mystical Pages
Wizard - 300 Advanced Mystical Pages
Blademaster - 480 Advanced Mystical Pages
Seeker - 180 Advanced Mystical Pages
Mystic - 180 Advanced Mystical Pages
When selecting the required Sage or Demon option please make sure it is the correct one for your cultivation preference.
Rarer level 92 Skill Books through Mysterious Pages
You can also acquire some rarer skill books through the usage of 50 Mysterious Pages at the Boutique Agent. One Mysterious Page can be created using 9 Perfect - Tokens of Luck.
These books are slightly rarer as they usually come out of FB99 so they are usually viewed as having a higher price in-game.
Level 99 skills
Level 99 skill books appear to only come from running the cube of fate. At level 90+ you receive a Page of Fate as a reward for winning the cube. With 30 pages of fate you can buy a book that gives you a random book among your level 99 books.
A page of fate can also be made out of 10 Book Credit Cards.
Level 100 skills
These skills do not come in demon / sage variants. For these you need 20 Chrono Pages and 10 medium ink to craft. Chrono Pages drop from the level 150 world bosses that appear around the world. They also have a small chance to drop from card bosses, mini world bosses, Treasure Map quest Map Five bosses and mobs in the past maps.
There may be some exceptional ways to get books via the Tournament or the "Hall of Blasphemy" but I am not sure if those are currently attainable.
Skills from Descent update (Morai skills)
This very nice List made on PWI forums BY:Asterelle,  should provide the info you need for theses skills.