Captured Spellbooks and Spell Research

I've been thinking about captured spellbooks and how they would impact spell research, particularly since one of the players in our game has snagged someone else's 1st level spellbook. Originally, it was essentially just supposed to be treasure in book form, able to be sold in any town that has buyers interested in magic. But I did tell the elf player, "there may be some additional spells beyond those on the standard 1st level spell list." I didn't say what spells, because I haven't decided which way to approach this:

• Randomly roll for four additional spells, per the creative variants in my previous post;
• Let the player make up spells she'd like to have and say "those are in the spellbook";
• Just add some spells from other published sources, like maybe some modified druid or cleric spells.

Whichever choice I make, this means the player will eventually do spell research... which means I need to know how a captured spellbook affects spell research. Originally, I was going to just convert the spellbook into  a sale value and use that to offset research costs. I figured the sale value of a spellbook is half the cost of replacing a spellbook: 1,000 gp for a 1st level spellbook, double that for each level above the first. But now I think this should only apply to spells that *aren't* in the spellbook. Spells in the spellbook should be easier to research, so I'd say that a spellbook with an unknown spell in it halves the research costs to learn that particular spell. That doesn't really affect M-Us who spend the minimum amount to research a spell, but those who spend more have much better odds: a 5th level M-U researching Fireball who spends 12,000 gp has the following odds of success:

• 30% by default;
• 40% with a captured spellbook that does not contain Fireball;
• 60% with a captured spellbook that contains Fireball.