As far as I can tell, there is no example of any other ancient Jewish book being composed the way the Documentarians claim the the Pentateuch was composed. The closest example is probably the Temple Scroll from the Dead Sea Scrolls. (See J. Charlesworth, The Dead Sea Scrolls, Volume 7: The Temple Scroll (2011); D. Swanson, The Temple Scroll and the Bible, (1995).) The Temple Scroll is composed in part from passages from Exodus and Deuteronomy, with many additions from the author/redactor. In other words it is the combination of two separate received texts (Exodus and Deuteronomy), merged with additional original passages to create an new book, precisely as Documentarians claim the Redactor of the Pentateuch combined J, E, D and P into a new book. But unlike the Documentarian claim that the Redactor combined the complete texts of JEDP, with little paraphrasing or harmonizing, the author of the Temple Scroll extensively manipulated his received texts. The author of the Temple Scroll reorders texts, omits texts, uses imitation archaic Hebrew style, changes verb tenses, conflates several sources into a single verse, has God’s speech turned to first person, substitutes words, and paraphrases some passages, while merely vaguely alluding to verses in others. Sometimes he quotes his sources fully and accurately, but in other passages he changes the text seemingly at will.
The most important point to note is that without the existence of the texts of Genesis and Exodus for comparison, it would be impossible to separate and reconstruct the original texts of Genesis and Exodus from the Temple Scroll alone. In this real-world, empirical example, with known source-texts and a surviving final composition, the author did not compose the Temple Scroll the way the Documentarians claim the Redactor composed the Pentateuch. Paraphrase, harmonization and synthesis are the norm in the Temple Scroll. The reconstruction of the “original” JEDP-sources from the received Pentateuch is possible only if the Redactor accurately copied all of the original four source-texts into his final synthesis of the Pentateuch. If the Redactor of the Pentateuch changed writing styles, reordered texts, omitted texts, changed verb tenses or person, substituted words, or paraphrased passages—just as the author of the Temple Scroll clearly did—the type of textual reconstruction Documentarians claimed to have achieved would be impossible.
The original JEDP sources of the Pentateuch can be restored only if those original sources were accurately reproduced in full in the final received Pentateuch. But it is a mere assumption—indeed, really an assertion. If the Redactor of the Pentateuch composed his work like the Redactor of the Temple Scroll, then the sources of the Pentateuch would be unrecoverable. Pragmatically speaking, there is no reason to believe that the Redactor of the Pentateuch did not use his sources precisely like the Redactor of the Temple Scroll used Exodus and Deuteronomy. If so, the claimed reconstructions of the original sources by the Documentarians are necessary erroneous, or flawed at best.