A Tall Man in a Low Land by Harry Pearson — A British journalist's affectionate, funny account of travelling through Belgium in the 1990s. Captures the country's understated oddness better than most travel writing. Still the best introduction to the place.
In Flanders Fields by Leon Wolff — A clear-eyed account of the Third Battle of Ypres (Passchendaele) in 1917. Essential reading before visiting the WWI battlefields around Ypres.
The Sorrow of Belgium by Hugo Claus — The defining Flemish novel, following a boy's coming of age under German occupation during WWII. Dense and rewarding; gives a feel for Flemish culture and memory unlike anything else.
Michael Jackson's Great Beers of Belgium by Michael Jackson (the beer writer, not the singer) — The canonical guide to Belgian beer, from a writer who did more than anyone to introduce Belgian brewing to the wider world. Useful before, during, and after any trip.
The Flemish Primitives (various art history titles) — Before visiting Bruges or Ghent, reading up on Jan van Eyck, Rogier van der Weyden, and Hans Memling pays dividends. The collections in those cities are world-class and easy to underestimate without context.
Tintin by Hergé — If you're going to Brussels, re-reading a few Tintin albums first is not a bad idea. The comics are rooted in mid-20th century Belgian life, and the Hergé Museum in Louvain-la-Neuve (an easy day trip from Brussels) is excellent.