Pul Dickson, The Dickson Baseball Dictionary (1989) has an enlightening entry for slider:
slider/slide ball 1. n A modified curveball that is rolled—or slid—out of the hand, rather than spun hard. It has less motion than a pure curve and breaks slightly but sharply just as it is crossing the plate. In other words, it starts out like a fastball and then breaks without warning, like a curve.
It creates a strong illusion, which once caused Willie Stargell to compare hitting a slider to "trying to drink coffee with a fork." ...
ETY[MOLOGY:] The origin of the pitch has been obscured by time, but the first pitchers to make a name for themselves throwing the slider worked in the 1930s. They were George "the Bull" Uhle of the Detroit Tigers and George Blaeholder of the St. Louis Browns. The term has been in use since the 1930s, but before then the pitch was known by other names. Two of these were the nickel (or 5 cent) curve and the sailer.
The point of Lewis's saying that "All the pitchers had sliders" (which is a gross exaggeration, by the way) is that the newly arrived player in question—Billy Beane—had come out of high school accustomed to seeing only three types of pitches: a high-velocity fastball, a medium-velocity curveball, and a low-velocity change-up. Now he was facing pitchers who could command pitches he had rarely or never faced in baseball games—including the slider, a pitch that looked like a fastball on its way to the plate but at the last moment broke out of its trajectory like a curveball.
Other pitchers had other odd pitches—cutter, splitter, forkball, screwball, palmball, knuckleball, slurve, etc. Lewis is using the slider to stand in for all such pitches, to indicate that being successful against pitchers who thew only fastball/curveball/change did not guarantee success in a league where most pitchers had one or another more-exotic pitch. This was Beane's experience as a professional player: his excellent batting performance in amateur baseball didn't translate into success as a pro. He lost confidence in his ability to play well at the pro level, and had a very disappointing career as a major league player.