Bargain @ $0.99 from Atheneum in Canada & the US (price should be the same at all the usual retailers):
From the Mixed-Up Files of Mrs. Basil E. Frankweiler by the late E. L. Konigsburg (
Wikipedia) her 1968 Newbery Medal-winning children's mystery/adventure book, illustrated by the author herself. This has its own
Wikipedia entry.
Also @ $1.99 each from Random House, assorted YA fantasy series novels:
Philip Pullman's (
ISFDB,
Wikipedia) His Dark Materials trilogy is on sale again, presumably to tie into the release of the new sequel:
The Golden Compass,
The Subtle Knife,
The Amber Spyglass
Tamora Pierce's (
ISFDB,
Wikipedia) Trickster's Duet duology starring the daughter of Alanna of Trebond, heroine of her Song of the Lioness quartet which I rather liked,
Trickster's Choice &
Trickster's Queen.
I've heard those weren't among the better adventures in her Tortall setting (I've only ever read the first a long time ago and it seemed merely okay), but it's a good price to try them at if you've childhood nostalgia, as is her short story collection, also on sale:
Tortall and Other Lands: A Collection of Tales