One major reason this show did so well, and then perhaps the reason for its only 3 series run, was the absolutely brilliant casting of Dylan Moran, Bill Bailey and Tamsin Grieg who in the case of the first two are best comics in Britain and Grieg is a great comic actress. But being so good means that they are also busy people and so this is probably why it stopped. No bad thing though as it means the show never tired and ended on a high, like all the best shows shoul, but don't always.
So the basic premise is simple, Bernard Black, played by Moran, owns a book shop called "Black Books" and hires Manny (Bailey) as his shop assistant. Fran (Grieg) is Bernard's best friend I guess, quite possibly his only friend. She used to own the shop next door but that seems to stop and the next 2 series she's just around, and doesn't seem to have a job. Basically this is the background for all the high jinks they get up in this shop.
Anyway, the other reason this show is so good is the writing. It's funny and sharp and the plots, although surreal at times, also seem like they could happen, and that's good comedy, cause we often laugh at what we recognise. (One thing I find is that I've actually been in shops that are simillar to the one in the show for it's chaotic look, so I realte with this.) It is also of course brilliantly executed by the actors.
At less than 9 hours long for the entire series 1-3 it's very easy to watch in a day. So you've got no excuses for not watching this as soon as you can.