I'm going to focus first on the Fifth Symphony, which is the first work of his to really grab my interest. This is a symphony in one movement. Sibelius wrote his last symphony in one movement. Whereas the Sibelius flows like an organic process, Rautavaara's symphony is like a communication among aliens on an icy planet, at least in the middle. It is not atonal; there is always melody going on, but the statements made are spontaneous and discrete. The opening builds with a series of ominous crescendos, leading into whirring sounds and the characteristic tympani motifs which surface in many of his later works. Should you be curious about where the final passage begins, it is around -4:21. This is one of the most breathtaking conclusions I've ever heard in a symphony.
I hope this is a starting point for learning more about Rautavaara. His more recent work sometimes sounds a little similar. There is a nasty Amazon reviewer who points this out in every single one of his reviews, which irritates the daylights out of me. But that particular individual hasn't stopped me from coming back for more. I'm already in too deep. I'm a big fan.
The First Symphony is good stuff. If Sibelius's First sounds a little like Tchaikovsky, then Rautavaara's First sounds at times like Shostakovich. His faster-paced movements are generally shorter than the slower ones. Just an observation. It applies here in the First. The Third is especially engaging: it has the feel of a Bruckner symphony, but with some unusual serial techniques employed. One of my favorites is the Sixth - Rautavaara uses synthesizer sounds to create strikingly ethereal effects.
Rautavaara's most popular pieces are the Seventh (which won a Grammy) and Cantus Arcticus, which features recorded bird song. He wrote an Eighth Symphony in 1999. Whether the world will see another, we don't know. I can tell you he is remarkably prolific in all genres, including opera. He wrote a really nice early piece for string orchestra called The Fiddlers (Pelimannit) in a fairly conventional style. I have heard a number of his concertos, many of which are very enjoyable and have been recorded by Vladimir Ashenazy and Richard Stoltzman, among others. You can find quite a few examples of the composer's work on YouTube, and when you're ready, you can purchase a few of his recordings through Amazon and other online sources. I certainly have.