Actually No, Jennifer Aniston Is Not Working on a Cookbook
We had a twinge of déjà vu upon reading Eater's post declaring that Jennifer Aniston is authoring a "series" of cookbooks with Jamie Oliver. Some quick Googling revealed why: Eater's news source is a Yahoo India story dated December 7th, but it's a a nearly word-for-word rerun of a National Enquirer (Ireland) story from back in June, which reported the purely speculative news that Aniston "is apparently keen to team up with [Jamie Oliver] and create a tome containing her recipes."
And between then and now there's been some debunkery: Gossip Cop followed up on the reports back in November, with Aniston's representative clarifying that the claims of her writing a cookbook were "completely false." In summary: No, Jennifer Aniston is not writing a cookbook.