Rangers Apprentice: The Early Years by John Flanagan.

Rangers Apprentice: The Early Years by John Flanagan is a great prequel to the rangers apprentice series. This book is set after "The Hibernian" one of the short stories in "The Lost Stories," and even without reading "The Hibernian" you can still enjoy the story. That is one thing that john flanagan does really well in all his books. The book explains some of the history behind parts of the main series, (one I would highly recommend reading) and is a good read as a standalone. This book is set from the perspective of Halt, a grizzled ranger who eventually becomes Will's apprentice in the main series.