While not strictly a sequel of Guardian, it is a continuation of the story in the same world, with characters that reference the first novel. And the story told here shows how rich the author made the setting, and how expandable the stories are that can be told here. This story has a much different feel and tone, much more action and swashbuckling, but still has the comfort of having the same rules, same places, and a lot of the same backstory as the original one.
That said, I think you can read this without having read the first one, and still like it quite a bit. Both books are fun reads, and I highly recommend them.