If I'm holding a slayer spellbook, do I still get the slayer bonus if I cast the spells from scrolls in my pack?

I think so? You can hold a partially filled book in your hands and get the bonus even thought the filled book is in yor pack.

Haven't tested this, but I'd agree in assuming the slayers would work. The principle is that holding the book gives your magery direct damage spells double damage, the method of casting said spells should be irrelevant.

I myself have a regular filled spellbook from which the spells are cast, and all the slayer books are practically empty.

This is something I could try myself...just never had, but if I am holding a slayer spellbook and fire off a necro spell, say poison strike, would I get the added damage. My thought would be no, but I was just curious if anyone has tried it.

From what I have seen, you get the bonus for holding it... and can use spells from your 'full' book in your backpack. Not sure if that's the way it is intended to work, but I have seen it, and on my mage, I carried my fancy book for several days using the full one in my pack until my crafter filled the fancy one with spells. My mods seemed to remain in tact with what the fancy book had.