Accompanists are typically pianists, sometimes organists or digital keyboard players, who play collaboratively with other musicians, most notably vocalists and chamber musicians, in rehearsals and performances. Of all the major music professions, accompanists are noted for probably having the most technically advanced sight-reading ability, as they are frequently employed for the specific purpose of accompanying auditionees, very regularly without ever seeing the sheet music before the accompaniment gig. Accompanists are employed in universities, in (some) dance companies, in choirs, and by individual soloists for rehearsal and recital purposes.
Some repertoire that accompanists tend to get very familiar with include German lieder, which are art songs that were popular in the 19th century, showtunes for accompanying musical theater artists, church hymns, choral music written with piano parts and choral music written without piano parts (to ensure accurate rehearsals), chamber music written for soloist and pianist, and the orchestra parts of concerti to accompany soloists in rehearsals. One of my friends regularly accompanies a dance company where she is paid to improvise the entire rehearsal. I want that job.
A collaborative piano degree has become the gold standard for pianists looking for careers in accompaniment, as opposed to regular piano degrees. Although technical ability and experience can outweigh the necessity for a collaborative piano degree, most accompanists today, especially ones in their 20s and 30s, have degrees in collaborative piano. Still, you can most certainly become a successful accompanist even if you have just a Bachelor’s or Master’s in Piano Performance, however the emphasis in repertoire is very different between the two degrees.
Accompanists are usually paid in the range of $30 – $50 per hour of rehearsal and recital, although some of the top ones can usually charge more. If salaried, an accompanist usually makes between $30,000 – $45,000, more if he or she gets to work with a highly established institution. Many accompanists will work both for a salary and supplement their income with independent accompanying work outside of their employment as well, with many of the top collaborative pianists holding positions at the world’s finest universities and conservatories.
Here is the only book I can find on piano accompaniment. It is geared towards accompanying vocalists and transforming a beginner accompanist into an intermediate accompanist. It seems to be a definitely good read.