I'm a huge fan of the Discworld Series by Terry Pratchett and have recently finished the 32 novels that are considered the "series". I suggest starting with The Color of Magic: A Discworld Novel if you want to check the series out.
There have been other books that take place in the Discworld universe and one of the more collectible types are the Discworld Diaries. They began in 1998 with Discworlds Diary Unseen University and later transitioned into less regular editions like The Discworld Almanac for the Common Year 2005. As far as I know, they only released in Europe making them a bit tougher to get a hold of here in the US. But, as of today...my collection of them is complete!
I have been looking for a copy of Discworld's Ankh-Morpork City Watch diary 1999 for a few years now and finally found one in great condition at a decent price on Ebay.
In addition to this, I found out today that The Folklore of Discworld is available on Kindle so now I'm fully set.