Not all book marketing firms are alike, and some are better than others. But selecting the right one for you as an author depends in part on what you're looking for and your personality. If you're running a search for how to market my book, listings for quite a few firms will come up. Each one will present facts and testimonials about their success and their approach to book marketing and publicity. But it would be best if you also thought about your personality and how you work best. For example, do you like working with one person or prefer a team approach? It indicates whether a larger or smaller firm is right for you.
Most book publicists are either a one-person operation or an agency with a group of marketing professionals. One advantage of the agency approach is the availability of many minds to collaborate on your project. But you may prefer a single contact person and a finite approach. Also, if you go with a one-person shop and the person is ill or not available on a specific day, your campaign grinds to a halt. You also want to look closely at a firm's (or person's) experience. Have they had success promoting books in your genre and if yes, do you like the details of the campaigns they have produced?
Generally speaking, individual publicists specialize in one or two genres, while larger agencies have professional publicists who work in many genres. Also, agencies have opportunities for cross-pollination among clients, and their work with other authors may connect to you and your book in some way – think expanded opportunities. Also, is it okay with you if your publicist works from a home or virtual office instead of a bricks-and-mortar operation? Either way, make sure you check reviews from previous clients, as well as reviewing the person or firm's track record to date.
Some other factors to consider before hiring a book publicity firm include your comfort level with emails versus phone calls for contact with your publicist. Established firms with offices are often more available by phone and while single practitioners and newer firms may be more reliant on email. Lastly, one of the most significant considerations is the level of control. Are you looking for a book PR firm to handle everything related to your book's marketing, or will you do some on your own? You need to have a comfort level with the level of involvement and control a firm will have over the process.