twilight: the apple. signifies the forbidden fruit/blood theme of the main story
new moon: the flower. just a random picture that the publishers picked out for the book
eclipse: a frayed ribbon. signifying that no matter how clean you try to make a break, there are always loose threads left behind.
breaking dawn: chess pieces. definitely means something, but we're not sure what yet!

you should definitely pick them up, jaitne. at the very least, just to see what all the fuss is about. it's really a very captivating story.