not a patent, unless you have a novel way to construct a greeting card, since patents are awarded for physical entities and innovative intellectual concepts [such as mathematical systems, scientific discoveries and such]
just the idea for the series and the writing itself is only in need of copyright registration, if you want to do that... otherwise, your automatic copyright will be in force anyway...
for all the info on both, go here:
United States Patent and Trademark Office Home Page
copyright info will be found in a link there...
hope this is helpful... if you want any help with the greeting card stuff, you can email me, as i dealt with biggie 'gibson' at one time, with a series of card concepts i'd come up with...
love and hugs, maia
maia3maia@hotmail.com