Upvote:2
A collection of his letters translated into English is sadly unknown to me.
The orignals are at
Besides the hundreds of manuscript volumes, the scientific-historical legacy of the two Bolyai brothers, which comprises almost 20000 pages, embodies an outstanding value. The Institute also holds an archive collection.
A few collections of his original letters seem to be printed in various books: bibliography
A few more transcribed and online are at