If you ever want to write a book about Mozart or Schubert, Otto Erich Deutsch [german link] is your man. His "Documentary Bibliographies" contain nicely cleaned up copies of all the contemporaneous source materialfor example, letters to and from Mozart, petitions by Mozart or his father for work, concert programs, testimonials written by people who witnessed Mozart performing as a child, newspaper announcements, journal entries, and much more. So in one volume, you have all (or practically all) relevant material about Mozart (or Schubert). There's no need to go any farther looking for new stuff (although I'm sure many people do).
Instead, you just read Otto's book, and then write your own.
Here is an excerpt from "Daines Barrington's Report on Mozart" to the Royal Society, 28 November 1769:
I carried to him a manuscript duet, which was composed by an English gentleman to some favourite words in Matastasio's opera of Demofoonte. The whole score was in five parts, viz. accompaniments for a first and second violin, the two vocal parts, and a base. I shall here likewise mention, that the parts for the first and second voice were written in what Italians stile the Contralto clef; the reason for taking notice of whic particular will appear hereafter. My intention in carrying with me this manuscript composition, was to have an irrefragable proof his abilities, as a player at sight, it being absolutely impossible that he could have ever seen the music before. The score was no more sooner put upon his desk, than he began to play the symphony in a most masterful manner, as well as in the time and stile which corresponded with the intention of the the composer. I mention this circumstance, because the greatest masters often fail in these particulars at the first trial. The symphony ended, he took the upper part, leaving the under one to his father. His voice in the tone of it was thin and infantine, but nothing could exceed the masterly manner in which he sung [...] When he had finished the duet, he expressed himself highly in its approbation, asking with some eagerness whether I had brought any more such music...
From pg 415 of his Mozart book, in a chapter with title "1791":
From the "Weiner Zeitung", 3 December 1791
New Music
At the Musikalishes Magazin [...] have appeared completely new works, fresh from the press, on the best paper, well and correctly engraved, viz:
From the opera die Zauberflöte by Herr Mozart:
Terzetto: "Seid uns zum zweitenmal wilkommen" for the pianoforte, 15 kr.
Duetto: "Bei Männer, welche Liebe Fühlen", 15 kr.
Aria: "In diesen heilgen Mauern," 10 kr.
On 3 December there was a slight improvement in Mozart's condition.
At 2pm on 4 December a kind of rehearsal of the as yet unfinish Requiem was held at Mozart's sick bed.
Count Franz Walsegg-Stuppach (see 2 April 1788) had the Requiem commissioned from Mozart anonymously in Spring, 1791, wishing to perform it as his own composition in memory of his late wife. Mozart had already tried out parts of the work with Constanze, Süssmayer and other friends at his home. On this occasion he sang the alto part to Schack's falsetto, Hofer's tenor and Gerl's bass.
On Monday 5 December 1791, a five minutes to 1 am, Mozart died.
Dr Closset had been called in during the evening of 4 December, but was unable to do anything more. The diagnosis at the time was "heated miliary fever"; the supposition today is that the cause of death was a "uraemic coma following a lengthy kidney disease." (Aloys Greither)
The book goes on for over 300 more pages, though. Otto explains:
The series of documents extends as far beyond Mozart's death as I consider useful, and for the first time brings together all the memoirs of him that are worthy of credence or consideration and were written or published during the ensuing 100 years.