None of them... I'm just enjoying the show as it is.
I've got one book about TOS, one book and a magazine about TNG series and a biography, though; I did read a few comics, some interviews, a few fanfics and I had a quick look to the technical manual, to some specifications; just being curious, but not learning every aspect of it.
Maybe it's just me, but I haven't got much money to get everything of the show, so I just get what I think is worthy.