I followed some advice in this thread (can't find the post now, has it vanished? But thanks anyway) and downloaded free sample chapters of each of the two books on Kindle. What I seem to have figured out about Cassie/Lexie so far is:
1. Lexie was the name Cassie gave to her imaginary childhood friend (who appeared at the end of the last episode- but is she still imaginary, like the wolf?)
2. Later, when in the police, she was working for Frank and needed an undercover identity. She used the name of her imaginary friend Lexie and padded this out to create a backstory for this person. Frank knows all about this. ( It appears that Rob knows too, but how?)
3. When the project was over she forgot about Lexie and resumed life as Cassie.
4. There is another girl who is almost Cassie's double. She happens to want to change her ID (not sure why).
5. This is the dodgy bit: Cassie surmised that someone has come up to this girl and addressed her as Lexie Mangan, which has prompted her to take Lexie's name, seek out her birth cert, create a fake photo ID etc etc. (What is not explained is how this girl knew that the "real" Lexie was not alive and using her own ID herself.)
So the Lexie who was killed was living under a false identity that she'd only had for a few years. It is now looking as though Daniel has killed her, or at least ascertained that she was actually dead, which is why he knows that Cassie is a fake. (Wasn't this an obvious flaw in the plan, if any of the housemates are suspects in the murder case?)
So those are my musings on the Lexie stuff - please feel free to amend or contradict!