Over at Musical Assumptions, Elaine Fine has posted a list of memorable memoirs by an assortment of musicians, including singers, string players, and of course the great librettist Lorenzo da Ponte.
Here I must step in and quote the great physician Gregory House: Everybody lies.
And when they're not lying, they're forgetting or just plain making things up or repeating stories somebody else told them as their own. You should read memoirs not with a grain of salt but with a metric ton of salt.
Elaine's list doesn't include the memoirs of either Wagner or Berlioz. One need only glance at the footnotes to modern editions of those memoirs, or any reasonable biography of the composers, to see how much is just plain wrong. In the case of Berlioz, it seems many of the errors are inadvertent, a matter of writing about the 1820s and 30s from the vantage point of the 1860s, though some are self-serving enough that they're probably lies. In the case of Wagner, he lies about a lot, coloring events to match his later perceptions or the image he'd like to project about himself.
Then there's John Culshaw's famous, and hugely entertaining, and very, very self-serving, Ring Resounding. I'll take just one WTF example: he mentions that Decca had recorded a Ring at Bayreuth in the mid-1950s, but it just wasn't satisfactory and thus hadn't been released. Well, that Ring cycle is now available, on Testament; it is the real first stereo Ring, with Joseph Keilberth conducting, and it is in many ways more satisfactory than the Solti, because it is much better sung. Self-serving, you think?
On a smaller scale, there are any number of lies in the Eva Turner interview in Lanfranco Rasponi's The Last Prima Donnas. She didn't sing Turandot 200 times, except in a world where 75 equals 200; she was probably singing in Portugal when Turandot premiered, so any claims to have been present are just wrong (or they're deliberate misrepresentations or deliberate repetition of a famous story as if she was there); her La Scala career, on which she traded constantly in retirement, consisted of six or seven performances (a single Turandot as a cover; a few Sieglindes and Freias in the mid-20s Ring cycle).
Everybody lies. Keep that in mind any time you're reading a memoir.