The book Fall From Glory tells all about the lack of influence the Guild has in Skyrim, and gives two possible explanations for it. The author is named as Nithilis Lidari, but in the book the author writes that they have spent two years infiltrating the guild. This leads me to believe that it is someone that we know, someone that we've grown to like and someone who we see every time we walk around the Guild's headquarters.
I have two suspects in mind;
Rune and Etienne Rarnis.
Rune
Spoiler
Rune's story is too perfect for a mole. He doesn't know who he is, and he never gives any actual evidence to back up the fact that he was a thief before joining the guild. All other members have explanations as to how they joined, or why they joined. Also the letter addressed to Rune entitled "No Word Yet." It says that someone with very good contacts has been looking for Rune's parents and they have seemingly "erased themselves from history." Also every spare coin Rune gets from the guild goes to finding his parents. The author of Fall From Glory states that he feels bad about doing the petty crimes, so if the author feels that way, he wouldn't keep the money around. Rune could be giving the coin away, or doing something else with it, but he sure isn't keeping it.
As for Etienne Rarnis I'm still looking into possible connections with other people as to the credibility of my claim, so far Rune is the prime suspect, Etienne doesn't have an alibi, whereas everyone else does, so he's on my radar. I'll update this when I've done a little more digging.
~Ra'Vash'Dur
(I wrote this as a letter to the guild leaders from my Kajit thief)