The library at MOE has a shelf near the door with used books for sale very cheap 2 - 10 aed each that they don't need any more, or have been donated and not wanted to put on their loan shelves.
Just to add, if you bought books at House of Prose and you take them back in good condition, they give you a 50% refund too!Bookworld has the same buy-back policy.
Feline friends do a twice monthly book sale at Arte.
House of prose in Jumeirah and in ACE.
To add to that there's an ARTE being held tomorrow at Times Square, you may find the Feline Friends stall there.
Lots - as a prolific reader i am a frequent visitor....
http://www.thenational.ae/arts-culture/books/a-guide-to-dubais-best-second-hand-bookshops#page2
my favourites are:
BookWorld on Plant Street, Satwa
Dubai garden Centre
Feline Friends book sales at Festival City