Music Fundamentals - A Balanced Approach, 2 edition

I'm happy to see even more books with titles like: Music Fundamentals,
but no more ...theory...

What I found weird is that the author describes the unison '1' as being an interval, but for there to be an interval, there must be something in the middle, one must have something like the Latin name itself says: lat .: Intervallum = inter (between, in the middle) + valum (trench, ditch, wall). The author should know that the 9th, 11th and 13th intervals also exist and are not 2nd, 4th and 6th octave intervals above, and the eighth '8th' interval shouldn't be called 1st above, because it is not an interval.
If there is octave interval, there are also: 9th, 10th, 11th, 12th and 13th...
The problem was that in the ancient times of music it had a basic math closing anything in module 7, which only worked to explain the basics of music fundamentals.

That is why philosophy is studied since the ancient Greek, than 'UNISON' is only possible when two notes, two voices are produced at the same frequency. Although unison at its root means only one sound [uni=one , then became union sound], but in musical empiricism it became part of the nomenclature of intervals to differentiate different intervals of the ratio of 1: 1 or 0 of the same height, but can still be perceived as originating from different sources.
The best historic example is that in the middle ages around 1570, when women sang the same note as the men and thought it was unison, but sometime later in history, someone discovers the parallel fifth, that in the rules of counterpoint and harmony since the baroque, it becomes: 'Bad Voice Leading', which was the greatest avant-garde revolution of that medieval time, when they realized that women sang an octave above men. By the way, unisonus is from latin.

In music, the word: interval is used even in the languages: German, French, Italian, Spanish, Russian, Japanese and also more than 50, being the same word from Latin etymology.

The word interval is also used for counting the time, in music means the time from one act of the opera to another act, or from one concert to another, in other words the same as pause. In written music refers to the relation between two sounds as to their frequency.

- (Time) Period of time between two musical events, also known as pause.
- (Mathematics) subset of a line.

Therefore correcting this author, only when two notes are produced:
Perfect intervals are only: '4, 5, 8'.
Unison: '1' , because there is no interval between them, that is in f0 which is the initial frequency, then from that duplicated frequency we establish any interval, but when there is only one note by itself, it is only an isolated note or a single frequency.

We can't even use the abbreviation 'P' for unison, because 'Perfect' refers to the perfect interval, which doesn't exist for unison, so we have to use 'U'.
So: 'U1, P4, P5, P8'

Only books in French, Italian, Spanish have been written correctly, because they understand the etymological sources correctly, since their own languages use the words of their own source. If you prefer, think about physics.