In an article for the BBC website, actor/screenwriter Mark Gattis pays tribute to the Doctor Who novelizations of his youth, which were produced by a British publishing company named Target.

"Scarcely anything was repeated on TV in those days... so, if you missed something, you missed it," Gattis recalls. "In an age before video and DVD, the Target novelisations were a chance to relive the television adventures. Many of the black and white 1960s stories had been wiped by the BBC altogether, so the books were the only record. Through them you could experience stories that had disappeared into the programme's folklore."

This was also true for American fans who discovered the show in syndication in the late 1970s and early 1980s, as they rarely saw anything predating the Tom Baker era; imported editions of the books were one of the few ways we had of fully comprehending the show's rich backstory. (Once, a long, long time ago, we even had a copy of Castrovalva that was signed by Sarah Sutton!)

"The Target Doctor Who novelisations were phenomenally successful," he points out. "They ran to 156 titles and the books sold millions of copies world-wide, becoming one of the best-selling ranges of children's books ever published." Virgin Publishing acquired Target around the time that the original show ended, and when the novelizations ran out, they published several original books. Once the BBC revived the show in 2005, the network's publishing division decided not to adapt the episodes but has continued to publish original stories featuring the characters.

(via io9, which has a whole lotta covers from the Target series on display)