In its earliest years Penguin published longer books in two volumes, each sold at the standard six-penny price, and with pages numbered consecutively to make it clear that the two volumes comprised a single book; Queer Street was the eighth such Penguin edition.
I assume that the title of this set of Penguins is meant to refer to a new trend rather than a genuine 1920's London location, although it is difficult to be sure as Queer Street is mentioned infrequently and obliquely - but it doesn't connote what it would today. I think Queer Street is an allusion to what is suggested as the post-WWI phenomenon of young adults moving out of their parents' home and into bed-sitting rooms in the central parts of London, sharing bathrooms and heating up their evening meal on a gas ring.
Euan Cartaret takes this step when his only sister is married, as until this time her role in the family had been the rather limited one of staying home and 'keeping house'. His friend Mona Fenwick is also living in a bedsit, although this is perhaps a more interesting and novel state of affairs because she is female. The life she leads in consequence is not an easy one: she needs to evade the unwelcome sexual advances of her employer, and she must nightly seek out men willing to pay for her drinks and her meals if she is to have sufficient left to pay the rent at the end of the week. Such men often want something in return, and so an easy virtue helps her cope with her financial dilemmas.