So, out of personal interest, I'm wondering where to go to access some of the primary sources on classical music. E.g. letters from composers, or contemporary reviews. Especially if anything is accessible online.
There's a few things online in bits and pieces, but most of what I use is in transcription in books, especially composer's letters, that's still the most common method of researching in letters, not online. If you can be more specific about what you are looking for I can probably help you hunt it down. But a few things I like:
Lots of Charles Burney's papers are digitized from the Beinenke Library Charles Burney's published books are also widely available online.
A lot of Mozart's letters to his father have been published, as have Beethoven's conversation books, Schoenberg's letters, and many, many more. Is there a composer or time period you're specifically interested in?
Many composers have written letters that have survived and of course the actual music itself can be a great source. But music history is long is there a specific time or composer that you are looking for it might help narrow down where to look?
Finding this thread super late, I know, but I'm really mystified nobody mentioned Oliver Strunks's Source Readings in Music History, which is THE standard collection of documents that you are describing. Not all of it is available in that Google books page, and it's not a cheap book, but it's got an incredible array of documents and it covers more or less the range of Western classical music.