At the Back of the North Wind is next week's text and it's available on Gutenberg for free (so no excuses!)
If you want to have a read ahead, the George Macdonald Society website is a good place to start. There are a few of his essays on here, note, in particular, The Fantastic Imagination (here: http://www.george-macdonald.com/etexts/nonfiction/fantastic_imagination.html ) which will be discussed next week.
Fantastic Literature: A Critical Reader by David Sandner is also very good and also discusses other key texts. http://books.google.co.uk/books?id=-AR9FEgly9wC&pg=PA64&lpg=PA64&dq=george+Macdonald+fantastic+imagination&source=bl&ots=AcJwxoe-NT&sig=Ep9oy7x-jbt_X4C85RgkLClfQ2g&hl=en&sa=X&ei=NbUSUd-TGZOS0QXgioGABg&ved=0CD4Q6AEwAQ#v=onepage&q=george%20Macdonald%20fantastic%20imagination&f=false
Webb, Jean (ed.) "A noble unrest": Contemporary Essays on the Work of George MacDonald. Newcastle: Cambridge Scholars Pub, 2007
Roderick McGillis is another scholar who has written extensively on Macdonald.
If you have some spare reading time then cosy up with The King of the Golden River, it can be found here: http://www.gutenberg.org/ebooks/701