one that you go back to after a while and re-read it.
I have read The Riddle of the Sands, by Erskine Childers several times, the book was published in 1903 and I find it interesting for its historical fact that was eventally leading to the first world war and quite atmospheric. As well as the classics The Anaeid, the Odyssey and the The illiad, which I like to read now and again. I also liked to read the Stephen Donaldson fantasy books The Chronicles of Thomas Covanent trilogies.