I have really noticed that. If you pick up a textbook on the New Testament today it will talk about all sorts of redaction criticism and gospels such as the Gospel of John being created in layers and not written all at one time. But it seems when it comes to mainstream books on Islamic origins, it seems to treat the Quran as a monolithic document with one author composed in its entire form at one time as well as taking sketchy Islamic tradition at face value.
Yes, but I think in fairness that partly reflects the fact that no real consensus has been reached by critical scholars. It's very difficult to explain the field's current status to laymen. So mainstream books still lumber along with the traditional explanations, almost completely ignoring modern scholarship.
It doesn't help that the three biggest sources of the 'composed in layers' view are Luling, Wansbrough, and Luxenberg. While their basic arguments are partly correct, all three have also taken positions that, in large part, are clearly incorrect and overstated (Luling's historical background is silly, Wansbrough gives much too long a time period for Qur'anic compilation, and Luxenberg is much too undisciplined and speculative). And that makes it very difficult to recommend any critical source on Qur'anic composition.
As I have said before, I think reading the Qur'an through Islamic tradition actually impairs people from understanding it. Better is to simply read what it actually says, bearing in mind the larger religious context of late Antiquity -- after all, the Qur'an was addressed to those people, and the Qur'an could not be more explicit about their existing familiarity with Biblical narratives. Especially useful is to throw out all the [parentheticals] used by translations, which try to read Islamic exegesis into the text. If you throw out all the baggage, what you see is a composite text in constant evolution, in which older forms are deformed and altered, while their meaning similarly undergoes evolution. I think the main such evolution is from brief anecdotes of anonymous monotheist preaching in the earliest materials to -- in my view probably the latest layer -- interpolations about an Arabian prophet named Muhammad who is the seal of the prophets, aka 33:40.
Incidentally this is part of the problem with Luxenberg -- while he often correctly recognizes that the Qur'an contains deformed versions of Christian antecedents, he fails to recognize that the transformation was largely complete by the time the complete Qur'an was assembled. So the "virgins of heaven" probably started out as a metaphorical extension of Syriac Christian descriptions of paradise, but by the time the Qur'an was completed, the references to "houris" really were specifically referring to celestial virgins -- even if careful linguistic analysis can discover that this is a distortion of a sort from more Christian ancestors. Some portions of the Qur'an may follow the older usage, but the newer portions have been Islamicized (compare the use of the term "Muhammad" -- same issue).
In its last phase of significant alteration, the Qur'an as we have it was reworked (in my view) to support the narrative of a divinely inspired Arabian prophet with a special Arab revelation, which had become politically expedient, and by the time this concept was written into the Qur'an via alterations, additions, and redactions, its text had been largely Islamicized (as Luling correctly noted). This must have happened relatively quickly, perhaps 30-60 years after Mo's death. But the materials which were reworked, bits of monotheist preaching, had been in existence for much longer still. This is why the "Mohammed" readings are so awkward and random from a grammatical perspective ... they seem to be imposed on a more basic text that anybody could have read to the faithful ("say this......") in connection with reading liturgy (again, Qur'an simply meaning 'lectionary' in its original Syriac-derived sense), just as anybody could deliver a generic sermon without it being considered a divine revelation to a prophet.
After it was partially reworked in this specific-prophet emphasizing fashion (which was probably a major state-sponsored project), there was still some significant scribal modification and alteration (such as the Sanaa I palimpsest shows), monkeying with the rhymes and verse divisions, but the level of variation at that point was more adding and clarifying bits of text rather than wholesale massive fabrications or deletions.