Get your music book professionally edited. Make sure your editor has experience working with music-related manuscripts, otherwise you may waste time explaining any unfamiliar musical terms or theory. Pay careful attention to any diagrams that show scores. A terrific reference for you and your editor is "Writing About Music: A Style Sheet from the Editors of 19th Century Music" by D. Kern Holoman. This book is a must-have reference because it breaks down editorial concerns specific to music publishing, such as how to hyphenate lyrics, cite works by other composers and how to describe musical compositions in text.
Find publishing houses that print books related to music. Websites are available that contain lists of houses (see Resources). Distinguish between houses that publish sheet music only and houses that print text.
Investigate the submission guidelines for each music book publishing company. Most publishing houses list guidelines on their website, but you may also request guidelines by writing to the publishing house with a self-addressed stamped envelope.
Format your manuscript according to the publishing house's submission guidelines. Pay careful attention to how the house wants you to handle diagrams and score representations (e.g., embedded PDF Finale-based file, Maestro font).
Complete all of the peripheral documents related to the music book according to the submission guidelines, such as the summary and table of contents. Use these documents to create proposal packages for each publishing house.
Write a cover/query letter for the music book. Be specific in regard to which genre the book fits (e.g., jazz, history, musician biography). Avoid saying the book will be useful to all musicians -- publishing houses want to see exactly what voids your book fills and that you have a concentrated audience in mind. Include basic information about the book in the letter, such as length and title. Also include previous publishing credits. If you are unpublished, include music credentials that verify your expertise on manuscript's subject matter (e.g., professor of composition at University B for 20 years for a composition book).
Send the packets to the publishing houses. Send a short, concise status inquiry letter if you do not hear from them in the time specified for replies on the publishing house website or other documents.