"He was a great guy who worked hard," manager Bob Geren said.
General manager Billy Beane and Geren declined to elaborate on Brown's reasons for retirement.
"It caught us a little by surprise, but then again, things like this, when personal reasons come up, they're usually a surprise anyway," Beane said.
Brown was a central character in Michael Lewis' book "Moneyball," which chronicled the A's approach to player evaluation. A hefty backstop from the University of Alabama, Brown led all college players in walks as a senior but wasn't well-regarded by many organizations because of his body.
In response to Brown's retirement, the A's signed Matt LeCroy to a Minor League contract so they would have six catchers in camp. LeCroy, a former Twins catcher, is expected to report this weekend.
"Certainly there are no promises for Matt," Beane said. "He needed a job. It's an opportunity, albeit a small opportunity."
LeCroy, 32, owns a career .260 average with 60 home runs in 476 Major League games. Known more for his prodigious power potential than his defense, LeCroy has a .981 fielding percentage as a catcher in the Major Leagues. Last season, he batted .194 for Triple-A Rochester with three home runs in 247 at-bats.
As for Brown, the A's would welcome him back to the organization if he decides he wants to return in the future.
"It's absolutely an open door," Beane said.
Harden session goes well: Starter Rich Harden threw his first bullpen session of Spring Training on Friday, and the results were encouraging.
Harden kept the ball down and showed a smooth delivery in delivering nearly 40 pitches to Kurt Suzuki.
"He looked like Rich Harden when he's healthy," Suzuki said. "I'm pumped I can finally catch him."
Harden and Suzuki have often talked about their desire to work together, but they have never formed a battery in a game.
"He's really good at getting me down in the zone, better than most catchers I've worked with," Harden said after his bullpen.
As for mechanics, Harden's work with bullpen coach and former roving pitching instructor Ron Romanick paid off.
"I feel like my command's a lot better than it usually is at this point," Harden said. "We haven't made too many big changes, it's more been smaller things and a lot of repetition."
Foulke sharp in bullpen: Keith Foulke acquitted himself well in his first bullpen session of Spring Training.
The former A's closer, who did not pitch last year, impressed the A's in a session before they signed him, and according to Geren, has gotten better since then.
Justin Knoedler caught Foulke and thought the veteran threw well.
"He hit his spots, he threw about 40 pitches ...[and] he had some decent life on his ball," Knoedler said.
Blanton to throw to hitters: Joe Blanton, the A's expected Opening Day starter, will throw to live hitters from a mound Saturday.
The batters won't swing, so Blanton gets the benefit of throwing from a mound without a screen in front of him, while the batters get to track pitches for the first time this spring. The A's plan to have all their pitchers do so this spring as another step to game readiness.
Due to rain Friday, the A's did not have pitchers' fielding practice at Papago Park.
Report card: Pitchers Angel Garcia and Santiago Casilla have yet to report to Spring Training. Garcia is absent due to "personal reasons," while Casilla has a "visa issue," Geren said.