convenient, equity, Jasher, just, meet(-est), + pleased well right(-eous), straight, (most) upright(-ly, -ness)

Boyd’s Bible Dictionary:

(upright). Book of, wholly lost (Josh. 10:13; 2 Sam. 1:18).

Concise Bible Dictionary:

A book only referred to in Joshua 10:13 and 2 Samuel 1:18, and of which nothing further is known. The quotations are poetical. There are several writings extant bearing the above title, but neither of them have any connection with the one alluded to in scripture.

Strong’s Dictionary of Hebrew Words:

Transliteration:
yashar
Phonic:
yaw-shawr’
Meaning:
from 3474; straight (literally or figuratively)
KJV Usage:
convenient, equity, Jasher, just, meet(-est), + pleased well right(-eous), straight, (most) upright(-ly, -ness)

Jackson’s Dictionary of Scripture Proper Names:

righteous