Thanks to Bob McManus for pointing out that that volume 1 of Beethoven's letters is now available on Project Gutenberg. There are some lovely letters such as this amorous note to Bettina Brentano, and this passionate one to Bettina von Arnim which includes this gem:

I afterwards took [Goethe] sharply to task for this; I gave him no quarter, and upbraided him with all his sins, especially towards you, my dear friend, as we had just been speaking of you. Heavens! if I could have lived with you as he did, believe me I should have produced far greater things. A musician is also a poet, he too can feel himself transported into a brighter world by a pair of fine eyes, where loftier spirits sport with him and impose heavy tasks on him. What thoughts rushed into my mind when I first saw you in the Observatory during a refreshing May shower, so fertilizing to me also!

Footnote 2: According to Bettina (see Goethe's Correspondence with a Child, II. 193), their first acquaintance was made in Beethoven's apartments.