Dirk Hayhurst is a very entertaining author who should not be underestimated.
He seems to have an intricate knowledge of the type of experiences and interpersonal relationships that the average reader and baseball fan would find interesting, and, in fact, I would go as far as suggesting that you probably do not have to be a baseball fan to enjoy his books - although it certainly does not hurt.
While very entertaining and full of amusing stories, Out of My League, focuses more on the budding relationship between Dirk and Bonnie rather than the locker room hijinks and experiences of teammates in AAA and eventually the Major Leagues. While baseball is never far from the plot, I'd say this book provides more of a focus on the personal life of a baseball player than The Bullpen Gospels. That said, it is equally as insightful as the original, albeit a little less humorous. It does provide fascinating insight into the self-doubt and depression that someone who many would think are on the top of the world can experience. That, in itself, is reason enough to give the book a read.
Overall, if you have not read the Bullpen Gospels, I would suggest picking it up first. If you have read Hayhurst's original book, I would highly recommend reading this one as well, keeping in mind that you are reading a different book and that your expectations should be a little different for this one.