A-Prefixing is a non-standard speech pattern that occurs in many rural areas across the United States, most notably in the mountains of Appalachia. Thus, it is a characteristic of this particular vernacular dialect. Compare these two examples:
SAE = They went hunting.
AE = They went a-huntin'.
As your book notes, some words ending in -ing take an a- prefix (pronounced as uh), attached to the beginning of the word.
We see this feature a lot in everyday speech, but you can also hear it in a lot of folk music, even country and western (and even blues) music. An old folk song is illustrative of this:
A froggie went a-courtin' and he did ride, a sword and pistol by his side . . .
This linguistic feature--like many others that characterize dialects--is significant because it illustrates that all languages and dialects operate by structural rules. The speakers of this dialect are not consciously aware of this rule, nor would they be able to describe it the way that linguists do; however, the rule governs how they speak. It is part of their linguistic competence.
We can therefore make the following claims about a-prefixing in AE speech:
Given actual examples of AE speech, linguists can begin to describe the rules that govern a-prefixing. Such rules are always complex, but here are some of the basic patterns to the rule. The rules operate on three different levels: Grammar, Semantics (Meaning), and Phonology (Sound).
Grammar (See List A):
They make money a-buildin' houses.
Phonology (See List C):
Historical Background on A-Prefixing
Hal McDonald notes:
Far from being a deviation from ‘standard’ English, as the non-native listener might perceive it, this verbal construction has its roots firmly planted in the bedrock of Old English, and is directly analogous to far more familiar constructions in modern English. The addition of an "a" prefix to "-ing" verbs is merely an evolved state of the Old English addition of the preposition "on" to a verb to indicate a continued action or state of being.
If an Old English speaker wanted to let you know where
he was yesterday when you dropped by his house and found it deserted, he
would tell you that he had been "on-hunting" or perhaps "on-fishing" at
the time. Over the following centuries, the unstressed "on" prefix was
simplified to "a" (a-hunting, a-fishing), and then dropped altogether in
most (but not all) variations of standard English. The quaint sounding
Appalachian "a" prefix, then, is historically every bit as "correct" as
is its absence in "standard" English. And for that matter, it is not as
entirely absent from the lexicon of standard English speakers. Any English
speaker who has ever said that he was "asleep" or "alive," for instance,
has used the "a" prefix, for these two words started out in Old English
as "on-sleep" (in the state of sleep) and "on-life" (in the state of life).