Joshua is the Book of Conquest. It details how God’s chosen people proceed into the land, and experience the blessings that had been promised to their parents. The conquest is left incomplete, however, as the volitional obedience of Israel was incomplete. Some tribes did well, some tribes didn’t do so well. Joshua & Caleb, the faithful spies from Numbers, are the faithful patriarchs in Joshua. Joshua is the first of the twelve historical books (Joshua-Esther).
The Hebrew title yeshua‘ is the shortened form of Joshua (Jeshua), from which the Greek name Yesous is derived. Hence, the Septuagint title is Yesous, and the English title is Joshua.
The authorship of Joshua cannot totally be proven. Jewish tradition is almost unanimous in ascribing the work to Joshua himself. Josh. 24:26 shows that he wrote the final charge, at the very least, and it is natural to accept that Joshua wrote the entire Book. The fact that Rahab is still alive at the time of writing, makes for a very early date (Josh. 6:25). The epilogue to the Book was probably written by Phinehas, much as Joshua most likely penned the epilogue to Deuteronomy.