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.