There is a little bit in Morgoth's Ring (HoME 10) page 418 and 419 of the Houghton Mifflin edition which mentions Sarumans breeding program, but otherwise UT and LotR are the places to go (I dont recall there being any additional info in HoME 6-9, which deal with the writing of LotR).
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Tar-Elenion
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