11. JOSHUA was an important figure in Israel history as well as in Holy Bible. Why is the author for this book unknown or uncertain.?

Answer: Sidlow Baxter explained: "It would seem that the book of Joshua is so named because Joshua is its focal figure, and not necessarily as implying that Joshua himself was its author. Jewish tradition does indeed ascribe the authorship to him. The probability is that while Joshua himself supplied the materials, these were arranged and supplemented by some scribe a little later, or it may be that Joshua contributed the substance of the work, while certain of the elders completed it. The really important thing to maintain is that the editor-author was a contemporary, having first-hand knowledge, or authentic documents, so that the work is really a product of the period which it records." (Explore The Book) On at least two occasions the text reports writing at Joshua’s command or by Joshua himself (Josh. 18:8; 24:25)

I agree with the above explanation. An analogy is that many great leaders had their speechwriters. History seldom records the names of the speechwriters but ascribes the words to the leaders.(G. Lee)