Daniel Christlein:

>I am looking for good and comprehensive textbooks on a basic level that
>even astronomers can understand and that are suitable for self-study ...

The one book I've read that talks about how a composer actually sits
and writes or thinks about writing is The Shaping Forces of Music by
composer Ernst Toch.  There's not a lot of technical info - like harmony,
orchestration, and counterpoint (to take the Piston trilogy) - but there is
a lot about basic rhetorical strategies, musical "shapes," and insightful
analyses of major scores.  The technical stuff you will have to learn
eventually, but for getting started (assuming you can read music), I
recommend the Toch.

Steve Schwartz