What is best format to use to put together a reflowable ebook that I intend to turn over to a formatter?

ebooks, epub, pdf, reddit, IFTTT,

It is a cookbook, heavy with photography. I was planning on doing text in a shared Google Drive (docs), and then uploading individual photos to the shared Google Drive folder. I don't want to use MS word, but I'm open to other ways. via /r/ebooks https://ift.tt/tiBMLkG