Philip Glass is the best-known living classical composer on the planet, and the type of music he helped give birth to – minimalism – is the most popular style of concert music the late 20th century has produced. Thanks in part to Glass, the awkward adjectives I've just used to categorize music have become increasingly meaningless. Glass has influenced David Bowie, Brian Eno and Laurie Anderson, among many others, and minimalism has absorbed the sounds and rhythms of rock, so who's to say any longer what's classical and what's popular, what's serious and what isn't?

But take a trip back to 1969 and get ready for a shock. In that year, Glass, following on the heels of minimalist pioneers such as Steve Reich, Terry Riley and La Monte Young forged a new style whose radical austerity thumbed its nose at every other music of the day, classical and popular. Glass' early works have long been cult classics, only available on limited-edition, out-of-print Chatham Square LPs that he produced in the early '70s. Now, they've finally been released on CD – and they've lost none of their raucous power.

When you want to start a revolution, you usually take an extreme position, and Glass did just that. Two Pages consists of a single melody that expands and contracts as it constantly repeats. Music in Fifths take two melodies, each elongating and shortening each endlessly repeated, and runs them in parallel motion. Music in Similar Motion and Contrary Motion add further intertwining lines but never compromise the severity of the structure. And Music With Changing Parts expands the scale to epic proportions – 62 minutes of unceasing repetition.

But things aren't as simple as they seem. All these early pieces, written between 1968 and 1970, unfold slowly over expanses of time, and all do strange things to your mind. At first, the relentless repetition can seem merely hypnotic. Listen carefully, however, and you begin to notice a wealth of subtly shifting detail. Soon, you realize that hardly any repetition is literal; although the pulse remains steady, the whirling melodies are constantly changing in length, and the colors are dancing like a ghostly aurora. Like some ecstatic devotional ritual, the music aims to transcend, to open the mind to new states of consciousness. Although it can be listened to for its mesmerizing, druglike effect, its real value is in heightening awareness, not numbing it.

Given the instrumentation of the Philip Glass Ensemble (in the '70s it consisted of electric keyboards, amplified winds and wordless voices) – and its slamming beat and high volume – it's easy to see why Glass' early minimalism was so influential. Glass, however, went on to other things – all of them bigger, if not better.

Over the next quarter century, Glass transformed his minimalist style into a vehicle for music theater. His first (and in many ways, finest) opera, Einstein on the Beach (1976), has been treated to a spectacular new recording that restores its full 3 1/2-hour length. Set to a text consisting primarily of meaningless numbers and syllables, Einstein was minimalism gone maximal. Since then, Glass' operas have grown more conventional; Glass himself, always prolific, has taken on too many potboilers, some of them unworthy of his name.

Which brings us to his most recent piece of music theater, Hydrogen Jukebox. It must have seemed like a great idea to take Allen Ginsberg's timelessly hip Beat poetry, create a panorama of America's social ferment of the '50s through the '80s, then set it all to six voices. But Jukebox turns out to be an awkward hybrid that dilutes both poetry and music. When Glass sets Ginsberg's texts, the result is a rhythmically stiff, singsong recitation that does violence to the poetry's unpredictable, boplike lilt. Things only get worse when Ginsberg simply reads his poems over Glass' accompaniment because the listener can never quite absorb both media simultaneously – one is forced to focus on either the text or the music. Occasionally, Glass' music (which, like Ginsberg's poetry, is soaked in pop culture) really catches the mood of the text, as in the final "Father Death Blues," set as a hushed, mournful hymn tune. But such moments are more the exception than the rule.

So what to make of Philip Glass? He has been dismissed as a mindless popularizer, criticized for compromising his ideals, and yet he continues to churn out music for a large, admiring public. Hearing his early works is a much-needed reminder that Glass, at his best, is second to none. (RS 682)

K. ROBERT SCHWARZ