Both Volume I (1778-1797) and Volume II (1798-1805) of The Letters of William Godwin, ed. Pamela Clemit, 6 vols. (Oxford: Oxford University Press, 2011-) are now available in searchable format on Oxford Scholarly Editions Online. This is a subscription database accessible via most university libraries. Please encourage your librarian to subscribe if you are unable to access it. Further volumes will become available on OSEO as they are published.