Public Ideas — I run a daily romance newsletter with over 2,500 subscribers.

a special Valentine’s issue with sponsors could be a nice one to plan ahead for

Have you considered a 'blind date with a book' feature? Give readers the opening paragraph, a few clues about the story and a link to discover the title. An independent author could sponsor a slot, but the format would still be entertaining for subscribers. It might even give you an interesting metric: how many people click before they know which book you're recommending?