Letter frequencies depend on corpus, so these vary. They also vary by language. http://www.bckelk.uklinux.net/words/etaoin.html is an example source. There are many others. Google is your friend.
Unicity distance is more accurately defined as the distance required to reduce the number of spurious keys to 0. Shannon defined it as an approximation of the amount of ciphertext such that the sum of the real information (entrophy) in the corresponding plaintext plus the entropy of the encryption key equals the number of ciphertext bits used. His work further showed that ciphertexts longer than this were reasonably certain to have only one meaningful decryption.
Unicity Distance of ASCII Text Encrypted with Algorithms of varying Key Lengths
Key Length Unicity Distance
40 5.9
56 8.2
64 9.4
80 11.8
128 18.8
256 37.6