Some mild spoilers.
Some of the "scrolls" can only be scribed by Dak'kon or TNO. The spells you get out of the Circle of Zerthimon are such spells. Ignus, for instance, will not be able to use those spells, and neither will Nordom.
There are no "classes" for spells. There are just "schools" like evocation, transmutation, enchantment, illusion, necromancer, and so on. All mages in PS:T are generalist mages, so there's no need to worry about a restricted school. There are also clerical spells, useable only by a priest. There is only one joinable NPC in game who is a priest.
To memorise an already scribed spell, make sure that you have free slots for spells of that level, and left-(or was it right?)click the spell. The spell's icon will now appear on the left side of the spellbook. Rest.
Intelligence plays a part in how many spells you can scribe. Dak'kon's INT isn't all that great, although tattoos will remedy that. Me, I used Dak'kon as a support mage, as I almost always play TNO as a mage himself. Dak'kon shines much better as a fighter (with some protective spells on plus tattoos that enhance CON/DEX/STR).
