To get your hands on these previews, it seems like you'll currently need to be a US resident, but you can get hold of them via iTunes or Amazon, simply by following the links. If the previews are sufficiently exciting for you to consider dropping some actual cash into the writers' coffers, then you can obtain all three titles from both Amazon and Barnes and Noble via the links provided on the Blizzard blog post. Naturally, as mentioned, Bloodsworn is currently only available for pre-order.
What do you think of these graphic novels? Is this your preferred format for absorbing World of Warcraft lore? You can also check out WoW Insider's reviews of Pearl of Pandaria and Dark Riders.