The Faerie Queene was published in two instalments during Spenser’s lifetime: Books I to III in 1590 and Books IV to VI (with a slightly revised version of I-III) in 1596. In a letter to Sir Walter Raleigh printed with the first edition of 1590, Spenser indicates his intention to write at least twelve Books overall, possibly extending to twenty-four. The Faerie Queene, already a massive poem, could have been four times longer. Evidence that Spenser was still working on the poem after 1596 is provided in a fragment, dubbed The Mutabilitie Cantos, published in 1609, ten years after Spenser’s death. For all of their narrative and thematic completeness, along with the sense of summary that they provide …
King, Andrew. "The Faerie Queene". The Literary Encyclopedia. First published 29 April 2005; last revised 30 November -1.
[http://www.litencyc.com/php/sworks.php?rec=true&UID=921, accessed 19 April 2015.]