Writer's business cards - what to write on them?

It should be what helps people identify and find you as a writer. Use the penname since that's what people will find your books under. Put your blog or website on there at the least, as well as any writing social medias (twitter, FB, etc). If you want to put a writing email on there, you can. I don't know if you need the titles of your books since ideally the other links should do the same job, but a picture of one or more of your books should be on there. Basically, if you give someone this business card, and they go home and sit down in front of the computer with it, what do you want them to enter into the search bar?

I keep it simple. First comes my author name, then my URL for the author site. Finally, a very short catchy blurb "This is not your world...". The card itself is a nice graphic with the letters as part of the artwork instead of overlaid on back. I don't put anything on the reverse side as everything I want them to see is on the website.

You need an email address on your card; preferably one that you use just for you-as-writer.

I wouldn't put a phone number or address. Name, URL, email address.