I have had a brief look around, and beyond what you might expect on Wikipedia, there doesn't seem to be a lot out there.
So, as usual, I am prepared to rush in where wiser people might tread more cautiously.
This series (if such it develops to be) comes with my famous no quibbles guarantee: if you do not find it immensely satisfying and worthwhile, you can get a full refund of exactly what you have paid for it, without deductions for admin or anything else.
I will also, as is my wont, make clear my credentials for writing an introduction to chant: I have none.
That done, let's launch in.
The first problem, perhaps, is nomenclature. You say plainchant, I say plainsong, you say Gregorian, I say Roman, let's call the whole thing off...
I don't want to get too precious about this, and most people use these terms fairly interchangeably; but what I am talking about is the Chant of the Roman Catholic tradition - as opposed, say, to Byzantine Chant, which is rather different.
The reason it is called plain is because of its simplicity compared to polyphonic music; in plainchant, only one note is sung at a time. There is no counterpoint, and there are no harmonies, chords, and so forth. It is strictly monophonic and can thus be sung accurately by one person. If more than one person is singing, they must sing exactly the same thing at the same time for the chant to work.
In terms of history, it is fair to say that the origins of Chant are lost in the mists of time. We know that the Jews at the time of Christ sang the psalms for example: as Christ and his disciples did at the Last Supper. We do not know how directly liturgical Chant descends from their practice.
One tradition says that all the Chant was dictated to Pope Gregory (who was pope 590 - 604) by an angel: hence Gregorian Chant. A more prosaic explanation is that he codified the Chant.
The earliest chant manuscripts are from the 9th Century, and the earliest fully notated manuscripts date from the 10th Century: The three earliest surviving books containing the year’s liturgical cycle are: Chartres, Bibliothèque Municipale 47, Laon, Bibliothèque Municipale 239 and St. Gall, Stiftsbibliothek Cod. Sang. 359. (Ref: here)
These contain neums rather than staff notation, making it hard to be precise about exactly how the Chant represented was performed.
Neums - from around the year 1000 AD: St. Gallen, Stiftsbibliothek, Cod. Sang. 390, p. 61 (www.cesg.unifr.ch)
By comparing these early manuscripts with the earliest extant square note notation, scholars have made significant progress in interpreting these neumes, but there is still much that is unclear, and plenty of scholarly disagreement about their interpretation.
The introduction of square note notation, placed on a staff that indicates the precise relationship of one pitch to another, was a significant development, and I will pick up the story at this point in a future post.