I started with the second book in the series Silver Flame then read the first--Blaze. I liked the second better :) The next one Forbidden was pretty cool too. I heartily recommend Pure Sin too it's a standalone but also features a very cool Native American hero.

The very coolest thing to history geek me is Johnson's footnotes. I don't think her later books have those. I imagine someone at the publisher said--"nobody wants that in their fiction".