I have been delighting in these this evening. They are five titles from the second series of the Ariel Poems, published by Faber and Faber in 1954 after a first series on similar lines in the 1920s. Each envelope contains one poem, printed and sewn into coloured card covers with a full page colour illustration by a significant illustrator of the day facing the first page. Often there are b/w illustrations also. I can't decide if my favourite is the art my Michael Ayrton or by Robin Jacques.
Lynton Lamb illustrates Sirmione Peninsula by Stephen Spender
Robin Jaques illustrates The Winnowing Dream by Walter de la Mare
Edward Bawden illustrates Mountains by W. H. Auden
Michael Ayrton illustrates The Other Wing by Louis MacNeice
James Sellars illustrates Nativity by Roy Campbell