"[George] Macdonald [sic] (1824-1905) [was a] Scottish novelist, clergyman and author of children's stories [who] was admired by many of his peers for his tender spirituality through his graceful poems and fantastical verse. They rank high among the classics of juvenile literature" (The Literature Network).
Lewis "regarded [George] MacDonald as his 'master': 'Picking up a copy of Phantastes one day at a train-station bookstall, I began to read. A few hours later,' said Lewis, 'I knew that I had crossed a great frontier'" (wikipedia). In his diary entry for January 11, 1923 (years before his conversion to Christianity), Lewis wrote, "I read Macdonald's [sic] Phantastes over my tea, which I have read many times and which I really believe fills for me the place of a devotional book. It tuned me up to a higher pitch and delighted me" (All My Road before Me, 177).
For more information on MacDonald, visit the George MacDonald Society.
I am not able to find Lewis's source for MacDonald's quoted statement "God is easy to please, but hard to satisfy."