How can I find out what books have been published about the horn? How can I find out what music has been published for horn?
You can perform the same searches as recommended under the Books or Recordings tabs above, but search OhioLINK instead of just MaagNET. That will find you the vast majority of materials. You can also use WorldCat or one of the books listed below.
Bibliography, not annotated, of 6,029 writings concerning brass instruments. A few sections focus specifically on the trumpet. See p. 88 for writings about the history and music literature of the trumpet, p. 268 for materials that pertain to pedagogy, study, and technique of the trumpet, and p. 390 for trumpet acoustics and construction. The author and subject index can lead you to more materials related to the trumpet.
Appendix C, beginning on p. 181, provides a classified bibliography of more than 4,000 works for horn organized into the following sections: (1) horn and piano, (2) horn with orchestra, (3) horn(s) alone, (4) horn(s) with other brass instruments, (5) arrangements for brass ensemble, including horn(s), (6) chamber music, and (7) horn with voices.
3-volume dictionary/encyclopedia of musical instruments. The entry on "horn" nearly 15 pages and is divided into the following sections: (1) General, (2) Technique, (3) History to c1800, (4) History from c1800, and (5) Solo Repertory. Photos and illustrations. Bibliography of approximately 50 writings.
Annotated bibliography of music for brass instruments. You can find music for trumpet on the following pages: Methods and Studies, p. 1; Trumpet Duets, p. 9; Trumpet Trios, p. 14; Trumpet Quartets and Larger, p. 16; Duets for Mixed Instruments, p. 25; Trios, p. 26; Quartets, p. 28; Quintets, p. 31; Sextets, p. 35; Septets, p. 36; Octets, p. 36; Nonets, p. 37, Brass Choir, p. 37; and Trumpet Solos, p. 40. Beginning on p. 61 is a lengthy bibliography of writings about brass instruments.