So the sun stood still, and the moon stopped, till the nation avenged itself on its enemies, as it is written in the Book of Jashar. The sun stopped in the middle of the sky and delayed going down about a full day. Joshua 10:13

David took up this lament concerning Saul and his son Jonathan, and he ordered that the people of Judah be taught this lament of the bow (it is written in the Book of Jashar): 2 Samuel 1:17-18

The reference to other books in the Bible, not found in the Bible, does not indicate a conspiracy to hide their existence. It fact, the Bible approves their existence, but does not support them as divinely-inspired. People wrote books back then, commentaries, essays, poems, songs, history books, books of warfare, books of national pride. The book of Jashar was such a book.

I found this written by Wayne Jackson (www.christiancourier.com)

The expression, “book of Jashar,” translates two Hebrew words that signify “book of the upright” or “book of the righteous one.” Apparently it was an ancient Hebrew hymnbook containing nationalistic songs. The Syriac translation of the text in Joshua calls it the “book of praises,” or “book of hymns.” The references in the books of Joshua and 2 Samuel would suggest that this collection of songs were compositions designed to celebrate great battles and notable characters in Israel’s illustrative history, as the nation prepared for the coming of the Messiah.

A text in 2 Samuel may suggest that the book was used as a training text for Israel’s military forces. The passage reads: “and he bade them teach the children of Judah the song of the bow: behold it is written in the book of Jashar” (1:18 ASV). One scholar, R.A. Mitchell, of the Institute of Mediterranean Studies (Berkeley, California), has paraphrased the text as follows: “He instructed them to train the Judeans in bowmanship, the training-poem for which is written in the book of Jashar” (Wycliffe Bible Dictionary, C.F. Pfeiffer, Howard Vos, John Rea, eds., Peabody, MA: Hendrickson, 2003, p. 884).

The book of Jashar is no longer extant. A copy, circulating under that name since 1751, has been determined to be a forgery.