This is such a cool wall art idea and easily customizable with what you have on hand! No postcards? Try using greeting cards (there are SO many amazing handmade/letter pressed/screen printed cards these days), gift wrap, patterned scrapbook paper, etc. -- the sky's the limit!
Jessica Jones from How About Orange had a boxed set of classic book cover-themed postcards that she didn't know what to do with, so she turned them into this awesome piece of wall art! Her tutorial is pretty straight forward and can be adapted to other materials, like the ones listed above, pretty easily. Check it out!