Don't hire anyone - no matter what they offer - who promises you they'll sell 'X' copies of your book. Every book is different. The best any marketing company or PR firm can do for your book is make potential readers aware of it.
Don't send out a newsletter just to send out a newsletter. One newsletter a year that is really interesting is more beneficial than 12 that are boring. If you write two or three boring newsletters in a row, your readers will start to think you write boring books.
We need to write books that publicists and marketers and booksellers and book club leaders and librarians and readers can get excited about. That have something about them that makes them stand out. That makes them shine.
With so many millions of titles available, the books that will get talked about are the books that make readers talk about them.
Nora Roberts, Stephen King, Lee Child and George R. R. Martin write wildly different books. Their writing, plotting and styles have little or nothing in common. But they all write books and characters that readers find appealing.
Don't add people to your subscriber list just because they once wrote you a note. Or once answered a note you wrote to them. Don't put your address book into your newsletter database. Let your readers sign up.
PR and marketing doesn't sell books. It gets attention for them. It sends readers to bookstores and websites to read a few pages.
All the marketing and advertising sells the book as what it is and hopes that the book will be displayed so that your readers can find it.
I know one writer who has been subscribing authors without their permission and sending out what she thinks are helpful advice sheets, but they come off as if she's a know-it-all. She thinks she's marketing herself and her work. All she's really doing is turning readers off.