I put off joining the Thieves' Guild so I could get the code book for the Fighter's Guild without a fight or by pickpocketing (which I couldn't do at low level).

Now I'm nearly at the end of the Fighters' Guild quests. I have to kill the head of the Fighters' Guild and his supporters. Thought it would be a good time to join the Thieves' Guild so I could use the Bitter Cup to get Fire-Eye to betray the assassins' guild, and that would be one less person to kill.

Apparently, that's not going to happen. I cannot join the Thieves' Guild. They have their standards, you know.

I'm wondering when exactly I screwed the Thieves' Guild over enough to be locked out. I've not gone about randomly killing anyone. Was it the code book?

Can you run through the Thieves' Guild quests and join the Fighter's Guild later, or do you run into an identical problem?

It's possible your skills/attributes don't meet their standards for entry.

Just obtaining the Code Book disqualifies you from joining the Thieves Guild. If you get the quest Sotilde will tell you why.
I'll be honest. I join both guilds and I don't take this quest From Eydis Fireeyes. There are plenty of quests from Hrundi and Percius Mercius to advance in the Fighters Guild without crossing the Thieves Guild.
If you do the Thieves Guild Quests you will get the Bitter Cup to bribe Eydis and she will not give you or hold you to the Codebook quest.

Ohhh....Outside the box. Very nice. Thank you!!!

And most of my stats are at 100. I wanted the Thieves' Guild for the Security training.

There's an easy way around this.
1) Join the Fighters Guild
2) Get the book from Sotilde
3) Join the Thieves Guild
4) Give the book to Fire Eyes

If you want to join the Thieves guild in your present game, open the console and type

pcmodfacrep 10 "thieves guild"

This gives you back the 10 faction reputation points you lost when you gave the code book to Fire Eyes.

I appreciate that, but it's just as well I "role-play" this through to the end. Percius Mercius at the Ald'ruhn branch of the Fighters' Guild did try to warn me something was off with this mission....

I think I prefer the Thieves' Guild, and will try out a better-fitting character next time.