I have a lot to say about this and related subjects, so in between two old-time music trips, I'm going to take some time to write about this book, Eno in general, and the idea of music as process.
While it sounds somewhat new-agey, the title is actually precisely descriptive. Tamm says that Eno is unusual in that his music is less about melody, or the horizontal progression of the music, than about textures and timbre and tone and the space the music is happening in, in other words, the vertical characteristics that don't relate to the music's development through time. "It depends to a large extent on the harmonics, or barely audible frequencies, that are stacked vertically" on the main note you're actually hearing. 
He describes Eno's growing up near an army base, where American pop and rock&roll would appear free of any cultural context, and presents Eno's music in context, or rather, in several contexts -- not only in the art-rock movement of the late 1960s and 1970s, but in the continuum of experimental music going back to Satie and continuing through John Cage, and in cybernetics and feedback work, about which I'll talk more later.
He seems to favor Eno's ambient work over his more song-oriented work, which is a bit of a problem. Many people (including myself on many occasions) dismiss Eno's ambient work as boring aural wallpaper, or group it with much of the new-age music it inspired. It's not boring, though, and it has roughly the same relationship to most new-age music as genuine essential oils have to car air fresheners.
But I like the fact that Eno enjoys doing his ambient work, but also twisted hard-rock songs, beautiful folk tunes, and producing bands from Devo to Talking Heads to U2 to world music artists. Tamm unfortunately dismisses a lot of this work, or looks at it only as it influences the parts of Eno's work that he likes.
For instance, he rightfully spends a great deal of time talking about Another Green World, certainly Eno's most famous album and perhaps his best. It's a good discussiong but sorely lacking in some respects. He completely ignores two of the album's centerpiece songs (as opposed to instrumental pieces), "Golden Hours" and "I'll Come Running," some of Eno's most beautiful work, and analyzes the album's last song, "Everything Merges With the Night" only in terms of its musical structure ignoring what I think are rather beautiful lyrics that close the album:
Especially combined with the very few lyrics of the album's opening track:
Rosalie, we've been talking all summer
Picking the straw from our clothes.
See how the breeze has softened
Everything pauses in the night.
They sound quite meaningful. Perhaps they're a commentary on the usefulness of words as communication (or of lyrics), but they're worth talking about. Perhaps Tamm is trying to be faithful to Eno's lack of enthusiasm for words, but that's Eno's problem; most listeners, including me, do like them, and they do matter.
All the clouds turn to words
All the words float in sequence
No one knows what they mean
Everyone just ignores them
Tamm generally seems biased against simple songs. He dismisses Eno's 1990 collaboration with John Cale, Wrong Way Up, as "amateurish ... silly, over-produced ditties," saying Eno "does not write real vocal melodies" because "he doesn't really understand the power of functional harmony to create, support and propel emotional movement."  Right. I don't either, but as beautiful as much of Eno's ambient work is, I much prefer his songs, with their simple melodies.
So he ignores the "amateurish" songs in favor of telling us things like "Eno's most common type of harmonic ploy is a repeating chord progression" (glaringly obvious once you get past his musicologist language), or spending an endlessly long paragraph analyzing the fourth ambient album, On Land, in painfully clueless musicological terms: "a rotating set of pitches alternately suggestive of an F-minor seventh chord and a Db-major seventh chord ... a static Mixolydian pitch set .. a spare use of pithces suggesive of the Phrygian mode .. a chromatic, unclassifiable pitch collection over a constant drone..."
I know enough about music technically to understand what every one of those phrases means, but I also know enough about music to know they add up to pretty much nothing. It's a classic example of failing to see the forest for the trees, or more accurately, mistaking the dots on paper for the picture. Even though Tamm acknowledges at one point that Eno is using "seemingly stray pitches drawn from a diatonic pitch set," even though he knows how Eno works and how Eno approaches music, he still writes as if the particular pitches have some kind of meaning, or were chosen deliberately or with some understanding (or caring) about the musical relationship between them. I found myself wishing Tamm would go back and reread the more insightful parts of his own book.
And much of it is indeed insightful, if pretentiously worded. He digs into the meaning of "ambient" and how Eno's ambient music is trying to acheive the opposite of music. And he looks in some detail (beyond mindless cataloging of pitches) into how he does it, how he creates landscapes rather than songs.
His main point is that Eno is interested in "non-teleogical music," in other words, music without a defined beginning and end. (To quote Eno's plain-language description, he likes making music that seems to be "just a chunk out of a longer continuum.") His ambient pieces often fade in and fade out, are cyclical without any real development, consisting of spaces rather than melodies.
They rely heavily on the use of extreme delay and equalization, used so subtly that a piece like "Discreet Music," 30 minutes of tape delay signal-processing largely created as Eno did other things, does not turn into aural wallpaper or New Age schmaltz. Removing and adding harmonics and emphasizing different colors in the sound spectrum, he creates a very deep sense of texture that varies very subtly. Eno says of his audiovisual installations that he likes to create the urban equivalent of "sitting bya river," and in this music he often does. Rivers don't change, on the one hand, and you could say that it's boring to sit and watch nothing happen, but if you look closely and are quiet enough there is a lot happening. And even if you aren't, the flow of subtle changes is soothing and beautiful.
Tamm identifies some very specific techiques Eno uses:
- Sparsity -- every element is clearly audible, and the "density of events" is low`
- Drone or background layer, usually with some kind of swooshing or gurgling sound, with delay and reverb creating a sense of space
- Long and deep delay
- Barely audible events
Thanks to his use of equalization and other treatments, he can even make single notes interesting, like the opening note of his collaboration with Robert Fripp, "The Heavenly Music Corporation," which lasts for more than a minute.
The sound is completely artificial in the sense that you do not imagine musicians playing it, nor could most of it be played live. And such a performance might be rather boring, consisting of Eno spool tape and turn knobs. In fact, Tamm says that much of "Discreet Music" was created by the tape loop system Eno set up, working on its own as Eno answered the phone and did other work.
But even though it's artificial, it is more human and natural than a lot of more "authentic" music, in the sense that it does dominate the listening environment and demand that you experience it on its own terms. Instead, it becomes part of the environment, and you can experience that way if you wish, or you can listen closely to it and hear the surprisingly beautiful variation and subtleties that are missing from most new-age music.
I'll write more about Eno's music in the next installment, but this book was quite interesting. Not perfect, but got me thinking. There are two points Tamm returns to in the book, both aphorisms of Eno's that are well worth thinking about:
"Repetition is a form of change."
"Every note obscures another."