So I have seen an awesome question about useful books for a E6 campaign. However, the question is for D&D 3.5e, while I am interested in playing Pathfinder, so I asked the same question, but about PF. It was closed as a shopping request, while the first question was not. Since the last close vote was put by a mod and there were 3 more, I believe all of those 4 voters have actually seen both questions, and the older one is not closed, even though they are pretty much identical.
I do not think that any of them has any of the problems related to shopping requests mentioned in Q&A is Hard, Let’s Go Shopping!, and both should stay here, as they are not real shopping questions.
But consider the voluminous amount of information you need to even begin properly answering a shopping question:
What is your budget?
Irrelevant, PF and 3.5e books are not that expensive, and content may be accessed for free in SRDs.
Where do you live?
Irrelevant, content may be accessed through Internet.
What are your preferences?
Specified in the question, to play Pathfinder E6 (or 3.5e E6, respectively) with more stuff than present in the the Core.
Which alternatives will you consider?
Obviously, no alternatives, the choice is specified in the question, it is PF under E6 rules. Or 3.5e under E6 rules in the case of the other question.
When do you want to buy?
I ask what books can I use to make my PF game better. The other question asks the same about 3.5e E6. It is not even specified if something is to be bought, not just legally accessed for free through SRDs.
Let’s say the question asker provided all that information. Fat chance, I know, but let’s pretend for a moment they did — and we were able to provide the perfect, ideal shopping recommendation to them. Even if that was the case, technology moves so rapidly that the best shopping recommendations will be utterly obsolete within a year!
Irrelevant, the books that were good for 3.5e in 2013 will still be good at 2017 or 2020, so will the books for Pathfinder.
So, both questions actually aren't shopping questions. The first one got a cool answer by KRyan, the second question could get one too.