single book title cannot be registered as a trademark with the USPTO. That rule is typically applied by USPTO examiners not just for books, but also for any "single creative work," including sound recordings, downloadable songs, downloadable ring tones, videocassettes, DVDs, audio CDs, and films. (BTW, the USPTO does not consider computer software, computer games, coloring books, puzzle books, and activity books as "single creative works" so you can register the title of these if there is only one.)
How many is a series? Apparently, two separate creative works is enough to establish a series according to the Manual of Examining Procedure. However, you will want to make sure that the submissions are significantly different in content -- for example, it's not simply the same book, but slightly revised. Also, the title (or portion of the title) used as a trademark must create a separate commercial impression (TMEP 1202.08(d)(i)) -- meaning that the size, font, and color used for the mark being registered for the series should be separable or different from the remainder of the title.