Birdoswald Roman Fort is part of Hadrians Wall, and is under the care of English Heritage who have published this guide book. It is a very interesting place to visit, but this rather brief book (just 40 pages) really brings it to life.
The first half of the book is dedicated to guiding you around the site and explaining the significance of what you can now see. There is a particularly good pictorial reconstruction of the fort and the main features are marked on this. Next we have a history of Birdoswald up to the 4th Century AD and briefly in subsequent times.
We obtained this book after our visit, but I would recommend that you get it beforehand and have it in your hand during your visit - it will really help you to understand the site.