Twilight
The cover of twilight, means the forbidden fruit... it refers to the story of Adam and Eve. Bella is the apple being held out to Edward, he could simply take a bite (there would be consequences of course) or with his self control, he could resist. The cover explains Edward's dilemma, however I don't know why the book is named twilight (other than when Edward and Bella were asking each other questions, it was twilight the time period, and Bella had to go inside.
Twilight: The time of the day when the sun is just below the horizon, especially the period between sunset and dark.
New Moon
New moon's cover is unknown... the tulip can be anything (I personally think it's Bella wilting away, because her life is empty without Edward...) however, the title NEW MOON, means it is the darkest period in Bella's life.
New Moon: The phase of the moon occurring when it passes between the earth and the sun and is invisible or visible only as a narrow crescent at sunset.
Eclipse
Eclipse shows a piece of ribbon that has been cut or ripped till it's hanging by a few threads. These are the ties Bella is trying to break off, she's trying to break off from Charlie, Renee, Jacob, etc. But she can't seem to totally cut them out of her life. I am unsure of what the title means...
Eclipse:
the obscuration of the light of the moon by the intervention of the earth between it and the sun (lunar eclipse) or the obscuration of the light of the sun by the intervention of the moon between it and a point on the earth(solar eclipse).