Wendy Wharam

I remembered reading this after seeing some illustrations that reminded me of the ones in this- really bad/ naive, as if they'd been scribbled with a biro!
The story is of mild- mannered Mr. Bootle, a bank clerk who lives alone but enjoys writing stories. One of the characters he writes about is an unusual creature called a Querk. Then one day, after spending so much time thinking about the Querk he turns into one!