I recently discovered a bookshop in Dorsoduro with a fantastic selection of English language books (an entire wall's worth!). Not a huge selection by newer writers, but based on the older forum, it seems like this place might have one of the best selections in Venice. I went to Staduim, Toletta and the San Polo Giunti so can only compare to those.
Cafoscarina Dorsoduro, 3259
30123 Venezia, Italy
This place was a savior for me for my upcoming 7 hour train ride . . .