1)The third item is Corinna's spellbook, and a woman named Penumbra has it. Unfortunately, she is being held prisoner in the thieves guild.
If you are already a member of the thieves guild, amble on down there and then talk to the guard at the far east. Go through all the speech options, and as you walk away, he'll leave his post. Then you can free Penumbra, and she will meet you back at the summoner's house to give you the book.
2)Well one benefit of joining is being able to get access to Penumbra so you can finish the quest. And there are other interesting things to do and find if you look around.
3)The only way to get to the vampire is to use the lockpick skill on the door. I think level 3 is required.