Hi Mark
Unlike some other places it's probably easier to find new books rather than second hand ones on Réunion.
The only second hand bookshop I know personally is La Bouquinerie, 5 rue Laferriere, St Denis (although I think there are two others in St Pierre and perhaps another one rue Felix Guyon in St Denis). It has a small selection of books in English, but you'd have to see for yourself if they have any sci-fi books - and be warned, prices there are not much cheaper than new. The other solution would be 'brocantes' (jumble sales/car boot sales) but you're unlikely to find books in English IMO. Most of the bigger bookshops (Agora, L'Entrepot, Librarie Gerard) have a very limited choice of books in English - again you'd have to see for yourself if they have anything that would interest you.
You have two other possibilities:
- a book swap website for used books like (I use it often to get hold of books I can't find elsewhere or that I don't want to buy new);
- if you want to buy new books use the website bookdepository.co.uk as it doesn't charge for delivery to Réunion.
HTH.