I'm often asked this question: How many books are your writing now?
I am only able to write one at a time, but there are several others in different processes. At one time, when writing Desires, I could be handling as many as 5 at one time--but only writing one.
The processes could be: 1.) drafting a proposal for a new one; this should be done immediately when finishing one book, waiting for an editorial okay. Or possibly before ending one bk, a writer will keep notes on a new one. 2.) CE or copyedits, which is the original manuscript that has gone through a copy editor for corrections and questions, and then returned to the author. The author then sees if they agree, or possibly adds more to the ms at that time. I may do that on my next set of copyedits as I am now in my second book of a trilogy and have more pinpoints to add to the first book. These additions would be labeled pg. 12a, etc. 3.) Galleys. The step after copyediting, when the pages with all the corrections are typeset into book format, usually left and right page, or perhaps a double space, numbered line set up. The author checks for continuity, spelling, etc. 4.) promotion of a book already published, which is intense in mass market. I calculate that mine consume around a month. 5.) writing a new one.
Juggling time is a major priority with writers and not easy. But I usually call my editors and ask them when I can be seeing the CEs or galleys, or page proofs as they are called sometimes.
This helps me schedule work loads.