Read this book about 20+ years ago:
Not sure what genre, perhaps YA fantasy, even though it was about a middle-aged and childless gentleman who lived alone and loved reading children's stories. From memory, he maintained a nursery in his house and longed to be a father.
He somehow embarks on an adventure across three different worlds inhabited by the characters he reads about. One of them was based on Winnie the Pooh and the Hundred Acre Wood, while another was a land populated by giants. I've never been able to recall this book's title.