Telekinesis triggers the default action of the object, so it opens letters rather than picking them up.
If you can not read it, maybe you are not close enough? Try getting closer or on the other side of a wall (if that is where it fell), clicking by the letter to move right beside it, and then reading and/or drag and dropping into your inventory.
If you can not get it, and can not re-load a recent save, a possible solution is below, though I would have to get the files from an old save.
Lost Items - guide and directory of inventory files
