Dick published more than 40 novels and over 120 short stories in a life that ended at 53. The output looks impossible until you see the machinery behind it.

The Direct Answer
Financial desperation, a machine-gun typing speed, and amphetamines.
He wrote nearly all his pre-1970 books on speed, by his own account in a 1975 Rolling Stone interview.

Paid by the Word, So He Wrote a Lot of Words

The pulp magazines that bought Dick's early work paid very little, so the way to make a living was to sell often. His first story reached print in 1952, and he never stopped needing the next check. He published roughly 30 stories in 1953 alone, and nearly as many the following year, because volume was the only way the math worked. Living on science fiction rates through the 1950s meant living close to broke, and he did for most of his career. As late as 1974 he reportedly earned only about $12,000 from all his writing combined.

The genre work wasn't the whole of it either. Across the 1950s he also wrote around a dozen mainstream literary novels, hoping to break out of science fiction and into respectable American fiction. Publishers turned nearly all of them down, and only Confessions of a Crap Artist reached print in his lifetime. He was writing constantly, in other words, whether or not anyone was buying.

Speed on the page came partly from speed in his hands. Dick had taught himself to type as a boy and hammered the keys fast. He also composed as he typed, drafting straight through instead of outlining first, which let a whole novel pour out in a couple of weeks when the pressure was on.

The Amphetamine Years

The other engine was chemical, and it belongs in any honest account of him. Through the 1960s Dick leaned heavily on amphetamines to sustain his marathon sessions. He said plainly that he wrote every book published before 1970 on speed. His most concentrated burst came in 1963 and 1964, when by several accounts he produced eleven novels, among them The Three Stigmata of Palmer Eldritch and Martian Time-Slip.

I'd like to say I'd have been able to do it without the amphetamines, but I'm not sure I could have done it without the amphetamines, to turn out that volume of writing.

Philip K. Dick, Rolling Stone, 1975

None of it came free. The drugs and the sleepless nights wore him down, his marriages strained and broke, and his money troubles never really lifted. Years of speed left a mark on his health that he couldn't outrun. He died in 1982 at 53. The output was real, and so was the price he paid for it.

For the wider picture, see the full Philip K. Dick writing routine.

What You Can Steal