Why Joshua is named as the author
The book begins where Deuteronomy ends. Moses has died, and the Lord speaks to Joshua, Moses' servant, calling him to lead Israel across the Jordan into the land promised to their fathers (Joshua 1:1-2). Joshua was present through the battles, the division of the land, and Israel's covenant renewal at Shechem. He was well placed to record these events.
Most significantly, near the end of the book Joshua gathers the people, renews their commitment to the Lord, and records what happened. The text directly says that Joshua wrote these words in the book of the law of God (Joshua 24:26). That is the clearest biblical reason for connecting him with the book that bears his name.
And Joshua wrote these words in the book of the law of God, and took a great stone, and set it up there under an oak, that was by the sanctuary of the Lord.
What did Joshua write?
Joshua 24:26 specifically refers to the words connected with the covenant assembly at Shechem. It does not give a modern-style title page saying, "Joshua wrote every line of this whole book." Still, it shows Joshua acting as a writer and keeper of Israel's covenant record, not merely as a military leader.
The book repeatedly presents Joshua as careful to follow the written instruction God had given through Moses. Before entering the land, he is told to keep the book of the law close and obey it faithfully (Joshua 1:7-8). Later, he publicly reads the law to Israel as Moses had commanded (Joshua 8:34-35). The book belongs in that continuing story of God's written word and covenant with his people.
This book of the law shall not depart out of thy mouth; but thou shalt meditate therein day and night, that thou mayest observe to do according to all that is written therein: for then thou shalt make thy way prosperous, and then thou shalt have good success.
What about the ending?
The last part of Joshua records Joshua's death and burial, then also tells of Eleazar's death and burial (Joshua 24:29-33). Joshua could not ordinarily have written an account of his own death after it happened. For that reason, many readers understand Joshua to have written the main body while a later, unnamed person added or completed the closing historical notes.
The passage itself does not identify that final writer. There is no need to force an answer where Scripture is quiet. The central point remains clear: Joshua's own writing is explicitly connected to the covenant record, and the book faithfully carries forward the history that began under Moses.
A helpful way to read Joshua
Knowing that Joshua was the principal author helps us hear the book as testimony from a leader who saw God's promises unfold. Yet the book is not mainly about Joshua's reputation. Again and again, it shows the Lord keeping his word, giving Israel the land, and calling them to serve him wholeheartedly.
Read Joshua with that question in mind: what does this event show about God's faithfulness, and how does it call his people to trust and obey? Joshua's final appeal urges Israel to choose whom they will serve, while his own household commits itself to the Lord (Joshua 24:15). That invitation still gives the book its living weight.
And if it seem evil unto you to serve the Lord, choose you this day whom ye will serve; whether the gods which your fathers served that were on the other side of the flood, or the gods of the Amorites, in whose land ye dwell: but as for me and my house, we will serve the Lord.