The life of a book-lover in Brussels is a tough one. I would ideally use the local libraries but their English-language selection is non-existent. English-language books are not routinely sold in the French and Flemish bookshops. There is a branch of Waterstones here, but the mark-up is obscene, making Amazon the only real option for affordable books
But hope is not lost. Ladies and gentleman, I give you: Bouquinerie Thomas.
I find browsing the shelves a curiously relaxing and ground experience. All the new adventures waiting dormant between their covers. It's so quiet and so calm. I couldn't ever trade my library for the equivalent in ebooks, I enjoy the experience of handling books as physical objects. Especially second hand - the previous readers and what they took, and what they left - messages, notes, bookmarks.
For Bruxellois - Bouqinerie Thomas is at 13 Rue Froissart, near Place Jourdan, and is open until 19h on weekdays. They only purchase books by appointment.