Look for a practice that treats the book as the start of a platform rather than a one-time launch, and ask who owns the mailing list and the content it builds. A good fit can explain how the work continues after publication day.
Two vetted practices list authors among the clients they serve: Mashman Consulting Group and Kathryn Holeton. They are based in Knoxville, TN and Little Rock, AR, and they are one consultancy and one independent consultant.
On how they work, both offer done-with-you work, where the client works alongside the practice. Of these, one works with clients remotely anywhere and one works both remotely and in person. Their working hours follow US Central and US Eastern time. Their price tiers, $$$ to $$$$, are shown on the listings on the same scale as every practice (what the tiers mean). Every practice here passed the same seven-criterion review, whatever its specialty, and the sample listings never appear on this page.