I got a quest from Eydis Fireye in the Fighters Guild Balmora to retrieve a codebook from Sottilde from the Thieves guild, but she won't give it to me and says I have to decide which side I'm going to take. I'm a member of both guilds and I want to stay that way, is there a way around this?

There is a way (that I can't remember) but I will actually try to convince you not to use it.. I did it in my first game and I have regretted it ever since. The game will become too easy when you level up and doing too many quest lines will make you too good too soon.. Are you a fighter or a thief? Do what is better suited for your character and the next time you play, do the other one. You will thank me later. Trust me on that.

I think whats so great about the elderscrolls is the fact that the play can do whatever they want. If the OP wants to get the book, let them.

That being said, theres two things you can do.

• Kill her. (do not just attack her. Successfully taunt her until she attacks you)
  
• Steal the book from her. (Save first! Use a chameleon spell/scroll to up your odds!)

I'd personally recommend the second one. Less concequence. But the choice is up to you.

If you want to avoid the quest in general, head over to another fighter's guild hall and pick up some more quests!

actually there's a way of finishing both quest lines. The point is that the fighters guild questline forks at the end. You can take the evil route with the current guildmaster or the good route with the former guildmaster Percius (n Ald'ruhn). If you side with Percius, then you can avoid the conflict with the Thieves guild. But if you want to finish both quest lines DO NOT simply kill people.

I stole the book, and everything works fine now.

How can I pick up more quests when I haven't finished one of their quests yet?

Each guild hall has a quest giver. The only one you won't be able to speak to is the one in Vivec as you need to be a higher rank in your guild.

Theres one in Wolverine Hall (attached to Sadrith Mora), Ald Ruhn, Vivec and as you know, Balmora.

Just a note: Some of the guild masters in the fighters guild are corrupt. If you get a quest, go talk to Percius in Ald-ruhn for advice. You do not need to do EVERY quest in the fighters guild, and as I recall all the corrupt quests are at the end of a given leader's quest line, so once you get a corrupt mission just stop working for them and move on to someone else. Also, if you get far enough in the Thieve's guild storyline, you can make Eydis skip the codebook quest (Even if you've already started it).

The Orc in Vivec, every one of his are corrupt when you pay attention to them. As to the OP issue, what Mikedzines says works very weel. Either one of those methods he posted work. I usually just taunt til she attacks and slaughter her and take the codebook. Saving before hand is a must however cause sometimes the little wood elf next to her gets mad when you take the codebook from her corpse.

1. Save, 2. taunt and slaughter, 3. take book. Quest done.

However if you want to join both factions, before you attempt this quest, be sure to already be in both otherwise they wont let you join