Knock, Knock, Who's There?

 was an activity book published by BBC Children's Books.

Publisher's summary
-What do we want?

-TIME TRAVEL!

-When do we want it?

-THAT'S IRRELEVANT!

Packed with a dimensionally transcendent TARDIS-load amount of jokes, gags and riddles, this is the official Doctor Who joke book! Remember: it's funnier on the inside!

Subject matter

 * Doctor Who jokes

Notable features
to be added