The ebook version is taken from translations by Lady Wallace originally published in the 1860s. It covers more than 450 letters, including famous ones such as the Heiligenstadt Testament (written for the world though addressed to Beethoven's brothers). A much more complete collection of letters was published in the 1960s (translations by Emily Anderson - around 1600 in total), but I'd say this earlier collection is a fine unless you're writing a PhD thesis on the man or you need to know about his laundry lists, etc. Many included here are letters to publishers and patrons, with quite a lot of wheeling and dealing, begging letters, etc.
Someone has taken quite a lot of trouble with this production, as there are active links from the contents list at the beginning to each letter. Music examples are included, no doubt a lot easier to read than in the original hand-writing! There is an active index, though this will only take you to the first letter under each heading (e.g. the first one to Archbishop Rudolf). One small criticism: each letter listed in the Contents takes three lines, so you have to Page Forward a lot to find individual items.