Problem: When critics see other books mentioned in the Hebrew Bible, they usually raise questions about the Hebrew canon: Were there many other holy books that never made it into the Bible? The Book of Jasher is mentioned here and in 1 Samuel 1:18. Is this an example of a “holy book” that never made it into the canon of Scripture?
Solution: A number of points can be made.
First, this is an example of a historical book that was not inspired. Because the Jews were a real nation and society, they had spiritual books (Scripture) and other books (history, poetry, etc.). It would be odd to say that this culture only had inspired texts. Obviously, they had other books in addition to Scripture.
Second, believers could discern inspired books from the non-inspired. The OT refers to many other books at the time that were not considered Scripture (Num. 21:14; 1 Kings 11:41; 1 Chron. 27:24). Paul refers to books that weren’t preserved for us in the NT canon (1 Cor. 5:9; 2 Thess. 2:2). Believers in the OT or NT did not choose which books were inspired; instead, they recognized which books were inspired.
Third, believers had a criteria for recognizing inspired Scripture. Only prophets could write Scripture in the OT. God gave the Jews two tests to discern if a prophet was from God: accuracy and doctrine. If a supposed prophet was inaccurate in any of their predictions, they would be killed (Deut. 18:20-22). Moreover, if they had false doctrine, they would be killed as well (Deut. 13:1-5). For instance, prophets were commanded to write for God (Jer. 30:2; Ezek. 43:11; Is. 8:1), and the historical books were written by prophets as well (1 Chron. 29:29; 2 Chron. 9:29; 12:15; 13:22; 20:34; 32:32; 33:19). OT prophets understood themselves to be inspired (2 Sam. 23:2; “Thus says the Lord” occurs 500x in the OT), and they understood one another to be divinely inspired (Dan. 9:2; Ezra 1:1; Is. 2:2-4; Micah 4:1-3; Jer. 26:18).
 It is difficult to know why Paul’s letter prior to 1 Corinthians was not Scripture. Would we have a 28th book to the Bible if it were ever found? Theoretically, yes. Providentially, no. If this letter did not make it to us, then God must not have meant us to have it. Therefore, perhaps this letter was not inspired Scripture. Instead, it was just a personal letter, unlike the rest of his letters (1 Cor. 14:37; 1 Thess. 2:13; 2 Pet. 3:15-16).