Driving Digest is probably the magazine you want - it is the most “generic” and wide interest of topics
The Whip is the publication of the American Driving Society - you get it with membership - does a lot of show reports and region reports and rules updates
The Carriage Journal is the magazine of the Carriage Assoc of America - and again you get it with membership
the Carriage Journal deals a lot in the history of driving and carriages as well as in pleasure driving, especially in Europe
I just found out in trying to ship some harness to Canada that postal (and FEDEX) rates to Canada are astronomical so you may find that cost for books is high
A book we liked when learning to drive was the Charlie Kellogg - Driving the Horse in Harness
it is an older book but had lots of good advice for single driving
http://www.amazon.com/Driving-Horse-Harness-Beginners-Manual/dp/0828903328
Many of the other older books like the Underhill book take you almost directly to pairs and 4-in-hand driving
I like this site for finding used books
www.abebooks.com
and I just noticed that you can ask the seller a question - so you could check on postage to Canada for books
Sallie Walrond is another author of a basic Driving book