They are the 2001 paperback editions, published by Houghton Mifflin for the Quality Paperback Book Club, featuring cover design by Grey Thornberry and original cover illustrations by Tolkien, as seen here: http://www.openculture.com/2013/02/disc
I came by the green Two Towers one secondhand, and would LOVE to complete the set.
EDITED (Jan 2016): Thanks muchly to everyone who provided helpful info! I now have all 3 books, and they look oh so pretty on my shelf. :)