This past weekend I gave a presentation on Dafydd ap Gwilym for a group, and thought I would put up a list of selected references here.
Dafydd ap Gwilym: Poems edited and translated by Rachel Bromwich. This is so far as I know the only bilingual edition which exists, with Welsh/English facing pages of the poems and Bromwich's translations. Its annoying features are that it does not include the full cannon, and the poems included are grouped by topics, making it (for me) harder to find a specific poem.
Dafydd ap Gwilym: his poems by Gwyn Thomas. A relatively brief introduction followed by annotated English translations of the complete poems in the order followed by Parry's Welsh edition. No Welsh except as examples in the introduction. Amazon lists this as not yet available; their link appears to be a reprinting of the 2001 edition I have.
Gwaith Dafydd Ap Gwilym edited by Thomas Parry. This is the standard Welsh edition (no English).
Aspects of the Poetry of Dafydd ap Gwilym: Collected Papers by Rachel Bromwich. A collection of articles by Bromwich on Dafydd and his poetry.
These are the books I would recommend to someone seriously interested in Dafydd's work. In addition there a number of other translations of selected poems, either by themselves or in anthologies. If there's interest I could list these in another post.