i would say the series by Tamora Pierce - the first book would be Alanna. They are wonderful for both boys and girls since they are about this girl trying to be a knight, a really nice fantasy. the books are relatively short and manageable and all the books are written in quartets.
also you might try the Arthur-books by Kevin Crossley-Holland. there are 3 and they are really wonderful
if they haven't read the Harry Potter books yet they have the right age to start, and should be able to read them on their own now.
also try finding some of the books by the swedish author Mats Wahl, I'm not sure how many of his books are translated into english but he has a series of crime-novels written for young readers and i think your 12 year old might like them. http://www.amazon.com/---/0374336091
this one is one of them :-)
The golden compass books by Philip Pullmann is also an option
i think that was what i could come up with. if they don't like reading make it fun for them. like make races and stuff - that will increase they're reading speed and they will think it is more fun. Also something like helping them find the right place to read. I was often told to read out loud for my teachers or parents, and was often told to sit at the table, or on the couch in upright position, but when i found out lying on the bed was way better i started reading so much more. Maybe their favorite place to read is on their bed or upside down on the couch... just a thought :-)