I'm not sure if there actually is a "big secret" between them in the books. But I'm only on like the beginning of the 6th book so maybe someone else can answer this better. :/

as far as i remember the two of them have not a big storyline..it somewhere mentioned that they might have met in boarding school,but i do not remember if it actually was recealed that the two of them ever were friends...so since the show is pretty much going into a different direction i don't think that you will find the answer for that in the books,guess you'll just have to wait until next week...

thanks

i have two left to read and georginas been and gone she wasnt anything special and her and serena didnt even talk alot

Yeah it just mentioned that they went to boarding school together. But it doesn't mention them being BFFs and partying and hooking up with guys together or anything like the show.

im pretty sure the only contact serena and georgina have in the books is in i like it like that when they have that party whilst skiing
they both go a bit mad whilst b almost sleeps with erik but nate walks in on them (fav bit)