When Sam Walker decided to join an elite fantasy-baseball league for the 2004 season, he chose Mariano Rivera, the Yankees’ star relief pitcher, in the first round.That turned out to be the only predictable move Mr. Walker made all year.

Mr. Walker writes a sports column for the Wall Street Journal,so he pays more attention to professional athletes than most of us. Still, he understood almost nothing about devotion to a sport until he began playing fantasy baseball so that he could write a book about the experience. The result is “Fantasyland: A Season on Baseball’s Lunatic Fringe” (Viking, 354 pages, $25.95).

Here is how the project unfolded. Because of Mr. Walker’s Wall Street Journal credentials and his book contract, he gained entry into an exclusive league known as Tout Wars: Battle of the Experts. He would compete against 11 other men.Mr.Walker wanted to test a theory – that, whereas other fantasy players based their decisions on statistics alone, his access to team clubhouses would give him a competitive edge in assembling his fantasy team and managing it through a 162-game season.

The competitive edge never materialized. At the end of the 2004 season, Mr. Walker’s team finished in eighth place in the 12-team league. Still, Mr. Walker’s access yielded fascinating scenes and anecdotes.

The book’s opening scene takes place in February 2004, in the spring-training clubhouse of the Minnesota Twins. Mr. Walker is trying to decide which players to draft when he meets his 11 competitors. Should Jacque Jones, an outfielder, end up on his team?

Mr. Walker saunters over to Mr. Jones and strikes up a conversation. The athlete notices that the author is carrying a book called “Baseball Forecaster,” written by the statistics guru Ron Shandler specifically for fantasy-baseball players. Mr. Jones does not fare well in Mr. Shandler’s book.

The ballplayer lifts the book from the writer’s hands. He reads the negative entry about himself. He is thoughtful for a moment.Then he says to Mr.Walker: “It’s sad that people write books like this. I hit .300 in the major leagues and still people are writing this stuff.”

Mr. Walker listens closely as Mr. Jones explains how he can rebut all the negative points mentioned in the book. He is impressed: “No matter how much the scouts or the statheads trash him, Jacque Jones will always find a way to succeed,” Mr. Walker finds himself thinking. “Not because he’s the most gifted ballplayer, but because he can’t bear to imagine the alternative.” He drafts Mr. Jones, whose batting average dips to .254 in 2004 from .304 in 2003.

After the regular season begins, Mr. Walker engages David Ortiz, the Boston Red Sox slugger, in a clubhouse conversation.Mr.Walker drafted Mr.Ortiz,too, but might trade him for Alfonso Soriano of the Texas Rangers due to a need for more speed on the fantasy team. Mr. Ortiz, laughing, warns against the danger of the trade. Mr.Walker makes the trade anyway, and ends up regretting it. When he encounters Mr. Ortiz again, the ballplayer asks how the trade worked out. Not so well, Mr. Walker replies. Mr. Ortiz, in the midst of a sterling season, enjoys the conversation immensely.

The laughs abound, but there is a dark side to fantasy baseball. This isn’t only a book about baseball; it is a book about obsession. And obsession can ruin lives. Mr. Walker documents how fantasy baseball causes marital problems, on-the-job distractions, and a misunderstanding of the real-life sport.

As Mr. Walker explains, statistical knowledge of player performance is not the same as understanding “the reality of running a baseball business.” What the fantasy fanatics fail to grasp, according to Toronto Blue Jays executive J.P. Ricciardi, “is that there’s actually money involved in Major League Baseball. Guys aren’t willing to play for nothing. You actually have to have a personality to deal with these players. You don’t just say, ‘Okay, come play for us. It’s a people game, too.'”