What do I do with scrolls and tomes?

[Shane]

I have a few of them in my inventory but I have no idea what to do with them. Halps

[Ǵ̶͓̂͑lí̴̤̀̄́tcĥ̸]

You will eventually meet an NPC that requires a key to get to, who is connected to another NPC you can meet in a nearby area. She's surrounded by rats, and the key requires giving an item also found in a nearby area, to a merchant in Firelink in order to be able to buy it. Being vague on purpose by the way.

This NPC can read these tomes, unlocking spells you can then purchase. Beware however, purchasing even one of the spells these tomes unlock can have its rewards, but its detriments as well.

There are a couple others too (some important). Just be careful what you do, exhaust dialogue. Or Google. Your choice.

[Nelyeth]

There are 3 "teacher NPCs", and each of them can read a kind of tome. You can find two of them in Undead Settlement and one of the in Road to Sacrifice.

[Sylar2jz]

They expand the inventory of teacher NPC's that you rescue and bring to the Firelink. Each NPC has a different set of items to give them to make them sell more of whatever it is they sell.

Same goes for Ashes, which the Handmaiden collects. She will sell some sweet stuff if you get the better ashes.

[Therealfather]

Give them to npcs at firelink

[XERXES_II]

There are actually 4 "teacher" NPCs.

Cornyx
Orbeck
Karla
Irina

If you care about what happens to the NPCs, I would recommend reading up on the last 3 before you mess up. Cornyx seems to be ok no matter what.

http://darksouls3.wiki.fextralife.com/NPCs