What transformations are, how they are learned and when they are cancelled, and where every Transform Sealbook comes from: monsters, raid bosses, the Adventure Guildsman, the subclass certification.
A transformation is when a player changes appearance by means of a spell. Transformations are combat ones, in which you can farm mobs and fight other players, non-combat ones, in which you can simply hang out, divine ones, and many more. To get a transformation you have to learn it. To learn one you need the transformation book (it is obtained by doing the transformation quest More Than Meets the Eye). You also need a Transform Sealbook scroll to learn a particular transformation and write it into the book. All transformations are added in the Ivory Tower, on the second floor, at the NPC Transformation Wizard Avant-Garde.
The first transformations were introduced in the Kamael update: those are the ones bought from the Adventure Guildsman in various towns; the Hellbound update added the raid boss transformations and the divine ones; the Gracia update added the drop of new transformations from ordinary mobs, but two of them were lost; the Freya update added more, but one transformation was never found.
We bring the scroll, choose the corresponding transformation and learn it.
A transformation is cancelled in these cases: if you pressed the skill that cancels the transformation, if you died, if you transformed by means of a talisman and took it off, if the transformation time ran out, or if you tried to run through water while transformed.
You can call a transformation if you have no summon or pet out, if you are not riding a strider or a wyvern, if you do not have the buff Mystic Immunity on you, and if you are not sailing on a ship.
On transforming, all the stats of the character change to the stats of the transformation. You can forget about duels while transformed. The subclass cannot be changed during a transformation, and neither can you transform at the Olympiad.
As I wrote above, there are also non-combat transformations that can be called with items: for example, the Yeti and the Buffalo transformations can be called with a talisman, and the Piggy with the item 
Pig Lollipop. The talisman, naturally, has to be put into a bracelet.
The crafters' transformation Golem Armor stands apart: in it crafters get the transformation of Doctor Chaos. It can be learned at level 83, not with a scroll but with the book 
Forgotten Scroll - Golem Armor, which is bought from the Olympiad manager for Olympiad tokens (the former noblesse gate passes), 52k, fame points (which are given for the territory wars), 26k, and adena, 12kk. This transformation lasts 1 hour, while the other transformations as a rule last 30 minutes.
And of course the racial transformation of the Kamael, Final Form, deserves a mention: it is learned after level 79 with the 
Battle Manual - Final Form. In this transformation the Kamael get their second wing, which they have dreamed of so much. True, this transformation lasts only 5 minutes.
The first transformation available to you is Onyx Beast, because this scroll is given to you after the quest. After that it is as it comes, because the ways of getting a Transform Sealbook differ from one transformation to another. The easiest to get are the scrolls that drop from mobs.
Transform Sealbook: Saber Tooth Tiger. Drop from mobs (all of them in the location Wall of Argos): Canyon Bandersnatch Slave (level 71), Canyon Bandersnatch (70), Canyon Antelope Slave (69).- Transform Sealbook: Oel Mahum. Drop from mobs (all of them in the location Forest of the Dead): Bone Grinder (level 70, both kinds) and Bone Scavenger (72).
- Transform Sealbook: Doll Blader. Drop from mobs (all of them in the location Antharas' Lair; these are the minions of Bloody Guardian, the mob needed in the quest Mimir's Elixir): Bloody Keeper (level 74) and Bloody Mystic (74).
- Transform Sealbook: Heretic. Drop from mobs (all of them in the location Stakato Nest): Male Spiked Stakato (level 74, both kinds), Cannibalistic Stakato Follower (74) and Female Spiked Stakato (74).
The Transform Sealbooks obtained as a drop from raid bosses (with these transformations the combat abilities rise considerably compared with the others, especially in the Anakim transformation): Transform Sealbook: Ranku, Transform Sealbook: Venom, Transform Sealbook - Kechi, Transform Sealbook: Anakim, Transform Sealbook: Demon Prince, Transform Sealbook: Gordon, Transform Sealbook: Zaken.
The transformation books that can be bought from the Adventure Guildsman for the raid boss eggs (one scroll costs 4 each of the items 
Memento Mori, 
Dragon Heart, 
Earth Egg, 
Nonliving Nucleus and 
Angelic Essence): Transform Sealbook: Dragon Bomber (in Aden), Transform Sealbook: Golem Guardian (in Schuttgart), Transform Sealbook: Inferno Drake (in Schuttgart), Transform Sealbook: Unicorn (in Oren), Transform Sealbook: Lilim Knight (in Oren), Transform Sealbook: Grail Apostle (in Gludin).
One transformation can be bought from the Black Marketeer of Mammon for 660k ancient adena: Transform Sealbook: Death Blader.
When you transform, all your skills are blocked and the skills of the particular transformation become available. Some of them are useless, some are not bad at all. All the buffs that were on you before the transformation stay with you.
There are also divine transformations, which a player can get after going through a subclass and raising it to level 80. They are obtained from the grand masters of the corresponding subclass during the certification. These transformations are learned on the main class and used on it: Transform Sealbook: Divine Enchanter, Transform Sealbook: Divine Wizard, Transform Sealbook: Divine Warrior, Transform Sealbook: Divine Summoner, Transform Sealbook: Divine Rogue, Transform Sealbook: Divine Knight, Transform Sealbook: Divine Healer.
You can neither hand over nor sell these scrolls, nor put them in the warehouse: only learn them yourself.
The transformations received on picking up a cursed weapon, Zariche or Akamanah, stand apart, but since these transformations do not depend on you and me, we will not talk about them.
In Gracia Final there was one more transformation; its scroll, Transform Sealbook: Vale Master, dropped from mobs on the fifth floor of the Tower of Insolence, but in Gracia Epilogue this transformation suddenly disappeared. Well, disappeared: the transformation itself remained, but the scroll stopped dropping from mobs.