Absolutely true, yes, you can buy some selections from them here.

Unfortunately a very large number of the books (believed to be the majority) were destroyed by Anton Schindler - a misguided musicologist and contemporary of Beethoven's.  His idea had been to remove from public circulation any conversations which weren't "worthy" of the great composer's reputation in posterity.

It wasn't a new idea.  Mozart's biographer, von Nissen (a Danish diplomat who had married Mozart's widow after a respectable time had elapsed since the composer's death) did the same thing, and used his diplomatic connections to track-down Mozart's letters and burn them, to blanch Mozart's posthumous reputation of disreputable stories and anecdotes.  Nannerl Mozart records being obliged to sell her brother's letters to von Nissen (he didn't disclose that he planned to burn them - he claimed they were for the biography).  Nissen's motive is alleged to have been that the Biography would provide an annuity for Constanze after his death (he was several years older than her).  At the Inquest into the death of Anna Selina "Nancy" Storace (Mozart's first Susanna, for whom he wrote a concert-aria "for Storace and me" with obbligato piano part, and a text which ends "no matter how far you may be, my heart will always be yours") Storace's maid testified that "foreign gentlemen had come, wanting the Mistress's letters from Vienna".  She added that "the Mistress threw the men out in an awful rage",  and the same night burnt the letters after drinking several cups of brandy, while shouting and cursing in Italian in German to herself.