Can I use these brackets to make wall mounted bookshelves?
2x10s will be fine with those brackets. The sagulator calculates about an 1/8" sag over the 6' span with a 500lb load and 2 brackets (anchored at sides), which is plenty acceptable - and 500lb is probably an overestimation for a single shelf. Shelving brackets are not intended to span the entire depth of the shelf either, but make sure the brackets are secured to the underside of the shelf properly