Here are some examples relating to music composed for a solo instrument (including voice).
As an illustration some resources for learning more about the flute:
Flute from Grove Music (need account login through university)
General overview of the flute family (with links to additional aspects)
Guide to flute technique (again with links to more information)
Examine the two most famous flute solo pieces in the repertoire: Debussy Syrinx and Varese Density 21.5
Very useful advice from an extraordinary flute player - Helen Bledsoe. Including advice for composers and an annotated repertoire list.
Detailed guides for performers and composers about the Alto Flute (with link to Bass Flute) from another wonderful player - Carla Rees.
Although old, Berlioz's Treatise on Instrumentation is an informative source, excerpts of which are available online with links to examples (but you'll need the Scortch browser plug-in to access those). More modern books on instrumentation and orchestration will also have useful material.
Each of the Sequenzas for solo instruments of Italian composer L. Berio is a useful example of the possibilities (aesthetic and technical) and capabilities of these instruments. For example his Sequenza XI for guitar.
Relevant not just in the domain of solo music - an interesting discussion of what is a musical idea.