The resources listed here will help in getting to grips with difficult poetry.
The Oxford English Dictionary which is essential for getting to grips with big words, obscure words and word origins.
The Dictionary of National Biography which is useful for getting a brief account of characters that Geoffrey Hill refers to although the quality of coverage is variable.
The AAAAAARG site which contains essential texts on Celan, Pound, Eliot together with essays by Prynne and is generally indispensable, containing works by Heidegger, Foucault, Adorno, Derrida and many of the other usual suspects (Harold Bloom, Hugh Kenner, Terry Eagleton, Stanley Fish etc.) You need to register but it's free.
Wikipedia has improved greatly in recent years in terms of quality and accuracy. Because of the way it's put together there is a lot of variation in the amount of information given. The article on Prynne is very short and doesn't say anything about the poems, the one on Hill goes into more detail. Wikipedia is more useful in following up on obscure subjects that poets refer to (eg 'sound particle' and 'soteriology') and does give a useful overview of the various critical theories.
Google is the most comprehensive and accurate search engine on the planet but there's no guarantee that it will point you to the best pages. This means that you can wade through pages of non-relevant dross before you get to the information that you want.
The Stanford Encylopedia of Philosophy is a growing resource which has good articles on Heidegger, Adorno, Bergson, Gabriel Marcel, Derrida, Martin Buber and others who have had an influence on difficult poetry.
The text section of the internet archive provides a treasure trove of work published up to about 1930.Has all of early Pound and Eliot and much more. Essential for those with an interest in early modernist work.
Grimm's Historical German Dictionary is only useful if you are fluent in German- this was the main dictionary used by Celan to wring as much meaning as possible out of his words. If you don't have German then the Chrome browser will translate (with variable results).
The second issue of Glossator is devoted to commentaries on Prynne. The first issue contains an essay by Prynne on "Tintern Abbey" and orther interesting pieces on Walter Benjamin and Raymond Rousel.
Jacket is Australian but that doesn't stop it being the very best resource on contemporary English poetry. It has many articles on Prynne, one excellent article on Geoffrey Hill and has a special feature section with each issue which focuses on one paricular poet. It's also very big, the current issue comes in at the equivalent to 900 pages. Its only annoying aspect is that it has had its search engine removed but there are instructions on the site as to getting round this..