Ultimate Magic has a section describing several pre-built spellbooks that PCs may use and some of them include preparation rituals. These rituals grant a boon to the caster if a minimum number of spells are prepared from the book. Here (Wiki) are the rules I am referring to.
Are there official Paizo rules for creating and balancing your own boons?
Some useful information I would like included in an answer if possible/applicable:
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Are there any rules that outline a PC adding a boon to a spellbook already in his possession? A 3rd party source is acceptable if there is no Paizo ruleset.
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Are there 3rd party sources that outline Pathfinder compatible (and preferably well-balanced) boons? This could include 3rd party sources for preconstructed spellbooks. (I would accept this as an answer if there is no official rule-set on the matter. Bonus points if it includes the aforementioned development and PC cost rule-set I'm looking for.)
I've done a Google search on the subject and have mostly come across dead forum posts making the same inquiry. Any sources on the subject would be appreciated.
Note: What I am not looking for are rules outlining custom spell creation and research.
Best Answer
Although I don't remember the direct sources, there have been a few books out there for d20 based games that discuss what I will call 'legacy items'. These items level over time with the players. They must put in the time with the item and in some cases something else, such as gold for ingredients, a sacrifice of some kind, or as a reward (of sorts) for a momentous occasion. There is also, generally, a minimum level for each enhancement level equivalent on the item. This approach could work for players with magic tomes and such as well.
I'll give an example to help clarify what I mean:
A player wants to play a samurai, and as part of his story, he want a blade that's been handed down through his family's lineage for generation. It has great rumors that the strongest heroes of the family have been able to harness the power of lightning through it.
It begins as a masterwork katana. Functionally, this is all it is at this level. At level 3, the samurai gets 1,000 gold in incenses, food, and other sacrifices, and offers them to his ancestors, hoping to please them. They 'are pleased' by the sacrifice, and thus afford the weapon a +1 bonus.
A year goes by, and although he continues to honor his ancestors it is more time for him to show his own heroism in order to awaken his blade. This comes in the form of killing a tyrant in the midst of a thunder storm. He is now a level 9 character, and his act of heroism has allowed his katana to gain the lightning burst ability.
This gives a good idea on how to approach it. Add powers to the item as the wielder unlocks the power. Use the level suggestions in the Ultimate Magic book as a reference for when they could reasonably gain that power, while adjusting for the powers that are already there. Simply give them the power instead of a new magic item in the same place. It gives that one item a much more personal feel anyway, and players tend to love that one choice item that level with them.