"to every people was sent a messenger..."

"And if all the trees on earth were pens and the ocean (were ink), with seven oceans behind it to add to its (supply), yet would not the words of Allah be exhausted (in the writing): for Allah is Exalted in Power, full of Wisdom." (Quran 31:27)

"To every people (was sent) a messenger [...]"
(Quran 10:47)

Evangelical missionaries tend to think that the Quran somehow endorses the 66 books of the Bible but I think they are thinking too small.

First of all, there were NO 66 Book Bible Christians in the milieu of the early Muslims. The ancient Christian churches all have bigger Bibles and accept many of the books Protestants consider apocryphal. On top of that, the Ethiopian Orthodox Church (which is probably the church with the most meaningful interactions with the early Muslims) have the largest canon of any Christian church (they have 81 books). On top of that, there are significant connections between Quranic statements and certain non-canonical texts (e.g. the Infancy Gospel of Thomas and the Arabic Infancy Gospel both mention the incident with Jesus (as) and the clay birds. The Acts of John, the Gospel of Peter and the Gospel of Basilides have non-standard / docetic takes of the crucifixion, etc.)

On the Jewish side, one could argue that some parts of the Talmud contain actual revelation. Also the Sefer Yetzirah could be the "Suhuf of Abraham" (as) mentioned in the Quran.

The Quran recognizes John the Baptist / Yahya (as) as a prophet and so perhaps the Mandaean scriptures contain revelation.

There are indications in some Islamic texts that Siddhartha Gautama and Zoroaster may have been prophets, and so Buddhist and Zoroastrian texts may contain revelation.

Luqman was a man known for his wisdom (who some people identify with Aesop) may have been a prophet. So perhaps the stories of Aesop may contain revelation. 

In a well-known hadith, the prophet (saaws) is quoted as saying "Seek wisdom, even unto China". Does that suggest that perhaps Lao Tzu or Confucius were prophets? Is the Tao Te Ching or the I Ching a work of revelation? 

And finally, if we take seriously the idea that "to every people We sent a messenger" and the claim that there have been 124,000 prophets (according to one hadith) while only 25 or so are named in the Quran, we should be very open to the possibility that large numbers of pre-Islamic religious figures may have been authentic prophets. I'm not a card-carrying Perennialist, but I would argue that a Muslim should have a charitable attitude, towards pre-Islamic religions. Even if their current forms may contain errors, those faiths may also be repositories of authentic prophetic guidance. We just need to sift the wheat from the chaff.