Grimoires

Authoritative books on magic are extremely rare, although many are known of, such as the Libram of Silver Magic, Nine Books of Nagash and the Necronomicon, in a pre-Gutenburg world they are almost never actually seen.  The books which are found are almost exclusively what are known as 'grimoires', often referred to as 'spell books'. The term 'spell book' is a misnomer, as these grimoires do not provide step by step instructions on how to cast a fireball. Rather they are a combination of research notes and personal journal which a wizard accumulates over their professional life.  Often they contain quotes and paraphrases of passages from the authoritative books, written down in whatever order the author encountered them.  Indeed, the contents of the great works are spread almost solely through these notes.

An arcane magic user who finds a grimoire has a resource to last his lifetime. By studying the notes of his predecessor (and copying extracts into his own grimoire), the magic user increases his own knowledge of the theory and Art of magic.  This provides him with insight to cast new spells.  Because this insight is coming from the synthesis of his current knowledge with the contents of the grimoire the spells learned from the same grimoire may be completely different for different magic users and the abilities of the grimoire's author have no bearing on what a reader may learn from the tome.  A chance turn of phrase in the notes of an apprentice may unlock a powerful spell.

Mechanics
The number of spells a character may learn from a grimoire is 1d4 + Intelligence modifier.

For each spell, roll on the following table to determine the spell level.  The player selects the spell they are trying to learn from the list of spells for that level.  If they succeed in the Spellcraft check, they have added their preferred spell to their spells known. If they fail the check, they learn a random spell of that level.  It takes two weeks of study to learn each spell.

Note that the DC is set so that in Pathfinder, a character which has Spellcraft as a class skill, has maximized their ranks in the skill and have the minimum intelligence to cast the spell have a 50/50 chance of learning their preferred spell.  It follows that a character cannot take 10 or 20 on the Spellcraft check, as the check is to see if the grimoire along with their current knowledge enables them to learn the desired spell.

Characters cannot learn spells of a level higher than they can cast.

Die Roll	Spell Level	Spellcraft DC
01-07	Special	N/A
08-10	0	12
11-28	1	14
29-44	2	17
45-58	3	19
59-70	4	22
71-80	5	24
81-88	6	27
89-94	7	29
95-98	8	32
99-00	9	34

Special: A result of Special indicates that among the entries in the grimoire, the character has found a note about an item or person of interest, such as the true name of a demon; a clue to the powers or location of a legendary artifact; or even a standard treasure map.