This is about the conflict. I have read the walkthrough on the uesp wiki but still want to clarify. Some said that its the giving the book to Eldis that messes your relations with the Thieves, so, it makes me wonder if I steal the book from Soltilde unnoticed and give it to Eldis, do I get any consequences or not? Which is the best option of the two, steal it while being already a thief, or as said in walkthrough, get the book from Soltitide and join Thieves before returning the book to Eldis? I would just prefer to steal it, peace of cake with Sneak 100, but wonder if the Thieves will hate me anyway?

I think the walkthrough method is the only way to have it not matter to the Thieves.

Face it, if you give the codebook to Eydis you are helping the Camona Tong destroy the Thieves' guild. You'll find out about that eventually.

If you want to join the Thieves' guild, join and do their quests. You will get a quest to find an item to bribe Eydis with. If you do this, she'll never ask you for the code book.