Thanks for writing, I'm glad you like it. I had to do some research to find your problem, as I wasn't aware of that possibility but I think if you go into Prefs/Behaviour you'll find that "Preferred output format" near the top left is set to MOBI and changing it to EPUB should get you the output you want.
The flip side of that is that you can output the recipes in whatever format you wish, which is nice.
I'm not aware of any way to insert into an existing book. I have played around with the epub merger plugin but the results are not nice. The table of contents functions seem broken and the resultant book is just one tacked straight onto the end of another complete with cover and TOC in the middle of the flow. If you are familiar with Sigil it wouldn't be that difficult to import the contents of one book into another but that would be a manual process.
I do have a couple of enhancements in mind for the plugin but I doubt that merging would be one of them, unfortunately. What we really need is a Calibre-wide way to do that e.g. a plugin that works. :-)