Simplest way: add a link to the recipe site as an EZpage. Both sites will have to be maintained separately and will continue to have separate databases. Check the Holzheimer's site in my sig and look for the "important links" box to see how it would work.
If you truly want to integrate the sites and put all the info in the Zen Cart database, you could create a category called "recipes" or "cookbook" and create the recipes with the product type "document general" which doesn't require things a regular product would require (price, weight, shipping). To see "document general" in use, check my nephew's site in my sig and is an example of what kobra was talking about. All the "products" there are "document general".
HTH!