Okay, contrary to how I view dogs (not as "furbabies," but as wonderful exquisite animals), I am way over the top when it comes to dog art. I have a vast general collection, but a couple of subspecialties: crate labels featuring dogs, Rie Munoz art including dogs, and dogs on snuff bottles.
The crate labels I think is the most fascinating. Crate labels in general have become sought-after pop art. Some of the artwork is quite high quality, and you can still purchase most of them for $100 or less. Most are much less, but some of the best are now exceeding $100.
A couple of my treasures are Collie and Greyhound depictions, both, I believe from the San Dimas Lemon Growers Association (I'd have to go upstairs to check, as they hang in my bathroom up there). I'm still on the lookout for Ruf'N Redy Lemons from the Upland Lemon Growers Association, depicting what looks to be an Airedale (I have a tiny magnet image) and Shepherd from the La Habra Citrus Association, featuring a Rough Collie. (For some reason, the citrus growing associations seem to do the best artwork.)
I used to collect mainly on ebay, but it's gotten very annoying there, as the segment has been taken over by people making cheap copies of original labels and selling them as "art prints." You have to wade through a lot of that to find a couple of genuine labels.
The Rie Munoz collection started with people who know of my collecting affliction giving me gifts. I think I now have a baker's dozen of her works or more, most with either her iconic black dog or a section of Alaskan Huskies, one an Old English Sheepdog, and one a tan dog.
The snuff bottle collection is small -- good snuff bottles can be quite pricey. It started because I worked on promoting an expensive book on snuff bottles (not dog related) and thought they were lovely.
I also have a variety of cigarette cards. There used to be whole lines of dog breed depictions placed in packs of cigarettes, with descriptions on the back. I have some framed and some in their original collection booklets. Again, the artwork is quite good, and seeing how breeds used to look is interesting.
You could choose to collect by breed. Scotties and Greyhounds are popular, Keeshonden and Portuguese Water Dogs are relatively hard to find. Or by form -- metal sculpture, ceramic, stuffed, mechanical, plastic. Or by era -- Victorian, occupied Japan, whatever.
There aren't a lot of good references for anything other than fine art (thanks to William Secord for that much, at least), and that's a shame. Most people can't afford to plunk down thousands of dollars for one picture, though many do.
But the vast variety of dog art out there certainly speaks to the dog's long-time place in society.