Artist Trading Cards are miniature arts of work made to a specific size - 2.5" x 3.5" (64 x 89mm).
These cards are never sold but are instead swapped.
There are a number of book about ATC's (specifically fabric ones) on the market e.g.:
Artist Trading Cards Workshop: Create, Collect, Swap by Bernie Berlin
1,000 Artist Trading Cards: Innovative and Inspired Mixed Media ATC's by Patricia Bolton.
Frieda Oxenham is giving two 1-day workshops this weekend on making ATC's, which should be extremely interesting.