The book series I remember most from my childhood is Freddy the Pig, which was actually a generation old when I discovered them. These books were the most incredible blend of semi-adult adventure plots with characters a child could identify with. I read every one which was available in our majestic Victorian hometown library with wooden floors and high windows back in North Carolina of the 1950s. In those days, libraries were solemn and silent places, full of endless stacks of books which could open any place and time in the world to you and encourage you to use your imagination.
As far as I know, none of these books (26 in all according to the Freddy the Pig website) were made into movies. But these classics still have a rabid following even today, a time when children do not learn to read and use their imaginations, and more's the pity.
http://www.freddythepig.org/whowas.html