There is a lot about Ennio Morricone's musical scores for the three "spaghetti westerns" directed by Sergio Leone in Christopher Frayling's bigraphy of Leone, Something to Do with Death
(London, New York: Faber and Faber, ©2000). If you can't find it at your local library
or bookstore, I could try to summarize the information here, but it would help if I knew more specifically what you had in mind.
You can also find some of Frayling's research on the Internet, for instance in an article on "The Making of Sergio Leone's A Fistful of Dollars
" that Frayling published in Cineaste
, available here:http://www.thefreelibrary.com/The+Making+of+Sergio+Leone%27s+A+Fistful+of+Dollars.-a064523916
Here's an excerpt, which reappears in the book on p. 155–156:
Morricone recalls that they 'worked very closely together in using music to "complete" certain characters on the screen, to complete them as types. For example, I tried to underline with music the ironic aspects of certain characters. Leone never really shared the American psychology, to which he manifestly preferred a more Italian reality-a down-to-earthness. Leone was born in Rome, like me. Certain characters in his films, the bad ones in particular, are very Italian, and even very Roman. But with stetsons on their heads. Nothing to do with American history, really, and to underline the irony and craziness of these Italian characters, I created an "Italian" sound.' Partly this was achieved through use of distinctiv e musical instruments which were unusual in a Western setting: 'I wanted to hammer out a kind of music which was more pressing, more troubled, more of a direct experience. So I used the Sicilian guimbard and the maranzano ['Jew's harp'], a Mediterranean instrument which is also played in North Africa and Asia.'
Morricone also enjoyed 'using instruments which resembled the human voice, like the flute and the violin' and 'using the human voice itself, solo and choral, as if it was a musical instrument [like in some forms of nasal folk-singing] because it is for me the most beautiful instrument of all with a sound attached to life itself.' Both the guimbard and the folk-singing were associated with the 'music of remote places,' which again was appropriate to The Magnificent Stranger [working title for what became A Fistful of Dollars]. 'When I began to compose for Leone,' says Morricone, 'I didn't think about it as writing specifically for that kind of film. With reference to the American Westerns which were the available "models," I wanted simply to use the idea of the escape to the prairie or desert and the expression of solitude. I wanted to put all this in music: isolated locations, a long way away from the noise and bustle of towns; I tried to re-create in my music this sense of wildness.' This is why whistling--as an 'expression of solitude'--seemed so appropriate .
For even more detail, there is a master's thesis by Emily Anne Kausalik titled "A Fistful of Drama: Musical Form in the Dollars Trilogy" available online here:http://etd.ohiolink.edu/etd/send-pdf.cgi/Kausalik%20Emily%20Anne.pdf?acc_num=bgsu1213650532
(This a a .pdf file.)