v. 1. Shorter poems / edited by Lynda Pratt, Ian Packer and Carol Bolton
v. 2. Roderick, the last of the Goths / edited by Diego Saglia
v. 3. Poems from the Laureate period, 1813-1823 / edited by Lynda Pratt ... [et al.]
v. 4. Fragments and romances / edited by Tim Fulford and Rachel Crawford.
OBNB doesn't sell books, but you may be able to find a copy at one of these websites:
Robert Southey : later poetical works, 1811-1838 by Robert Southey. ISBN 9781851969593. Published by Pickering & Chatto in 2012. Publication and catalogue information, links to buy online and reader comments.
