[Radek]

If you already have the quest then, well, the things aren't good. Either you fail the quest or you get kicked out from the Thieves Guild. Sottilde will not give you the book no matter how much you bribe her if you are a member of the Thieves Guild.
If you have a saved game before getting the quest then there is a possibility to save your membersihip and advancement in both the Thieves Guild and the Fighters Guild. There is some "gentleman" Jim Stacey in Vivec, which have a quest for you: bribe heads of the Fighters Guild so that the guild will protect the Thieves Guild. One of the bosses to bribe is Fire-Eye from Balmora. Bribe Fire-Eye and she will skip the Code Book quest and behave as if the quest has been finished successfully. In the other words, when it comes to the Code Book quest, stop your advancement in the Fighters Guild until you have Fire-Eye bribed, and then continue advancement in the guild. The Code Book quest threatens no more.
I am not sure what happens if you already have the quest, whether the "gentleman" will help. Perhaps, bribed Fire-Eye will consider the quest finished successfuly even if you fail it (by not bringing the book), perhaps not. I don't know.