In order to assimilate a piece of music more fully, I would first study the score, play it, hear it and observe it by myself. However, I would then desire to consult a high-quality resource that would give me deeper insight into the music. Two composers I am very interested in, Schoenberg and Stockhausen, both published writings on music, which are rich and rewarding. But what about Beethoven, and Bach? Are there any excellent writings on music that will help me to appreciate the music more deeply, in the way that a literary critic and analyst can bring to light hidden structures and ideas in a book that fascinates us (history of the author being sometimes noteworthy but secondary)? Are there any canonical, distinguished music critics or analysts? Someone who understands music as deeply as composers do themselves.
I believe you are looking for the discipline of musicology.
Start with the New Grove Dictionary of Music and Musicians, and then follow the bibliographies from there. Also, ask a reference librarian how to search for academic journal articles and books in musicology. In the Library of Congress labeling system, these books are in the ML category, literature on music.
One recent book on Beethoven that is highly regarded is "Beethoven Hero" by Scott Burnham. On Bach you might try "Bach and the Meanings of Counterpoint" by David Yearsley. There are thousands more books and articles on these composers.