I suppose that when this book came out in 1988 as Hollinghurst’s debut it would have been shelved in the “Gay Literature” section of your LFBS, but as his reputation has grown, and particularly since winning the Booker with “The Line of Beauty” in 2004, his work has started being read by a mainstream audience. The most startling thing about this novel for a “general reader” is the explicitness of the sex scenes. If such things offend, then steer well clear. Sadly, this will also lead you to missing out on a brilliant, superbly written novel.
Most of the book is narrated by a promiscuous gay man called Will and details the London gay scene of the pre-AIDS early 80s. The anti-hero is also aristocratic, so I read the book as being about the consequences of having no responsibilities; he cannot have a family and he has no need to work. At the start of the novel, Will is good company but clearly an ethical hollow man, but following a number of revelations he has learned some sense of decency. Not a lot, mind.
Incidentally, here’s what the Observer profile of Hollinghurst said about it, which provides a useful contemporary commentary:
"The Swimming-Pool Library… was described by Edmund White as 'the best book about gay life yet written by an English author'. Exhilaratingly libidinous, full of pre-Aids pleasure, it appeared with a tremendous sense of shock. The novelist Philip Hensher was 22 when it was published and recalls its impact: 'I remember coming down from university to London to buy it as soon as possible. It was extremely important to my generation: before that, you couldn't imagine a gay novel about gay life appealing to anyone else.' "