I think we can reasonably infer that the work Lewis is referring to is _The Works of Aristotle the Famous Philosopher_ , which, per Wikipedia, is a sex manual and book of midwifery, falsely claiming to be the work of Aristotle.
In other words, a work of pretentious smut. Just the sort of thing that Lewis liked to rail against.
So, the answer is: Smutty bookshops.