The one that you bought simply because you liked the look of it, not because you knew anything about it, or had read rave reviews or word of mouth on it, but simply took a punt and discovered a bona fide unknown classic. Well, there are a few, but the one I had in mind to kick off this thread is 'Moscow Stations' by Venedict Yerofeev.
I'm pretty sure this was one of those I pulled out of a pallet back in my remainder bookshop days, it was published by Faber, and something about the picture on the cover - a train in the shape of a bottle of vodka - struck me as amusing, so I bought it. And read it in pretty much one sitting. It's a simple story of an alcoholic, recently fired from his job, who decides to take a train journey to Petushki to visit his family. On the journey, he basically rants on and on about whatever comes into his head at any given time, whilst getting more and more drunk, to the point that when he does get to Petushki, he's asleep and misses the station.
At the beginning of the book, he claims that despite having lived in Moscow all of his life, he has never seen the Kremlin. And it also contains one of my favourite lines, when, having fallen asleep again earlier in the journey, he notices that he has less vodka than before, and amiably but vaguely threateningly takes it up with the man sitting opposite him - 'Without moving a muscle, he started to make a run for it.'
Just to confuse matters, the book was originally titled 'Moscow-Petushki', but has also been translated as 'Moscow Circles' and 'Moscow To The End Of The Line'. There are further complications with Yerofeev's surname, which has also been variously anglicised as Erofeev or Erofeyev.
Anyway, I bought this book, and then raved about it to people, but there was one slight drawback - it had gone out of print. A few years later, when working for another remainder bookshop, I was on a buying trip to a warehouse in Amersham and came across a dozen copies, which I promptly took for my shop, and pushed onto whoever I could until they ran out. And that seemed to be it, until last year, when I noticed a new (Faber) edition on a table in the Waterstones in Deansgate in Manchester and realised it had been republished. So now I can recommend it again to anyone with the hope that they should be able to pick up a copy fairly easily!