Hey, there are a lot of questions about books today! Although I was aware of the presence of professional scribes in this Islamic tradition since the 8th century, the warraqeen, I hadn't heard of such an assembly-line style of production. However, I did find within Jonathan Bloom's book Paper Before Print: The History and Impact of Paper in the Islamic World an explanation for how Islamic methods of reproducing manuscripts could have led to the production of many more books than pre-printing press Western methods.

Bloom highlights the impact of the oral tradition of Arabic and its necessity of ensuring a chain of transmission back to the original author. He claims that the reproduction of books was not primarily a written venture but rather an oral one - the author would recite his work, often from memory, within a mosque. Scribes who were listening would then record what the author was dictating, creating an official copy of the work.

I mentioned in my comment earlier today on why the printing press didn't immediately take off in the Islamic world the idea of definitive knowledge, or that it is not only important what information says, but where it comes from. This chain of transmission was extremely important in the Islamic world. Bloom mentions that for a manuscript to be considered an official translation, the scribe would have to read back the work to the author who would then certify that it is authentic and free from error. He adds that it was important to prove not only that a scribe heard a book from a specific source but also that he read it back to an approved source. With this double-layered verification, the chain of transmission was protected.

As arduous as this process may be, it has the definite advantage of ensuring that official reproductions are free from error and accurately portray what the author meant, a form of quality control if you may. Once a reproduction was certified as correct, it could then be read in other Mosques to circles of scribes to repeat the process. Bloom writes that

"in contrast to the situation in medieval Christendom, where a single scribe made a single parchment copy of the single parchment manuscript on the desk before him, one author in the Muslim world could generate a dozen copies from a single reading, and each of these authorized copies could generate another dozen. Within two 'generations' of readings, well over a hundred copies of a single work might be produced". (116)

He also mentions that authors could often also "double-down" and treat the recitation for quality control as another chance to have new scribes record the work, speeding up the process even more!

So although this isn't a traditional assembly-line in the modern, Western sense, it can be seen as a system in which books were rapidly reproduced and a certain level of quality was ensured. This can also explain how libraries in the Islamic world could number in the hundreds of thousands of manuscripts while the largest European libraries of the medieval ages were limited to only a couple thousand. I hope this starts to answer your question!