Tolkien's actual writings, drafts and sketches that were the source material for the Silmarillion are found in The History of Middle-Earth (excluding the volumes that deal with the writing of LoTR). To be more precise, the Silmarillion draws primarily on such writings as the Quenta Noldorinwa (from Vol 4 - The Shaping of Middle-earth), the various Annals, and the Quenta Silmarillion (from Vol 10 - Morgoth's Ring and Vol 11 - The War of the Jewels), along with some other writings. So, to examine the original material, I strongly suggest the HoME series.
The actual creation of the published Silmarillion is documented in Arda Reconstructed: The Creation of the Published Silmarillion. http://tolkiengateway.net/wiki/Arda_Reconstructed:_The_Creation_of_the_Published_Silmarillion
In The History of Middle-earth, Christopher Tolkien documents in amazing detail the development of the work of his father that would become the Silmarillion. However, save for an occasional hint here and there, he fails to show the final step: his actual creation (several years after his father’s death) of the published work, with the assistance of Guy Kay. As he points out in the foreword to The War of the Jewels (the second of the two volumes of The History of Middle-earth that covers the “later Silmarillion”), the source materials that he used in that task largely have been made available “and with them a criticism of the ‘constructed’ Silmarillion becomes possible. I shall not enter into that question. . .” (p. x).
The purpose of Arda Reconstructed is to “enter in that question.” Kane documents the changes, omissions, and additions that were made to Tolkien’s work by Christopher Tolkien (with the assistance of Guy Kay) in preparing the Silmarillion for publication, and traces how the disparate source materials were used to create what is in essence a composite work. He compares the published text with the source texts contained in the volumes of The History of Middle-earth (as well as other works such as Unfinished Tales of Middle-earth and Númenor, The Children of Húrin, and—in one case—Tolkien’s letters) and identifies patterns of major and minor changes made to these source materials that result in the reconstruction of the finished text.