Whether it be Joshua or Yehoshua (the Hebrew word for "Joshua"), the title of the Book of Joshua is a misnomer. That's right, we said it. The Book of Joshua is really a story about the Israelite nation, not about a man named Joshua. This book is like Exodus—it tells the events of the Israelite people. In fact, Exodus is more about Moses than Joshua is about Joshua. Go figure. Just for kicks, try saying Yehoshua five times fast. We promise it's fun.
So why name a book after one dude if the story isn't really about him? Answer: because he's important. Joshua is the man but not in a Y the Last Man kind of way. Although the book is really about the Israelites, Joshua is the guy that gets it all done. And more importantly, he's the guy God talks to. Joshua is the living vessel of God in this book, the all-important connection between divinity and humanity.
The Book of Joshua is not meant to tell a story about an individual, but instead is written to explain how an individual's connection to God is essential. Joshua is the bridge that connects the Israelites to God, and for that, he gets his name on a book.