According to this report - and contrary to what had often previously been thought - the monograph crisis isn't so much about a decline in the number of monographs that are being acquired by libraries because said libraries can no longer afford them due to the high and rising costs of journal subscriptions. Nor is it about the impact this state of affairs has on the kind of monographs that are being published - more short academic/trade books, textbooks, introductions and reference works selected for commercial reasons; and fewer original, specialised research monographs chosen on the basis of their academic quality and value. Nor is it about the consequences of all this for the academy, and for early career academics especially. No, the monograph crisis is said to be more about the number of monographs that are being published. And since the latter is apparently growing in the UK (although it's worth noting that the term monograph is often used quite broadly in Monographs and Open Access to take in edited collections, critical editions and other longer outputs such as scholarly exhibition catalogues), then one of the report's conclusions is that it's not appropriate to talk about a monograph crisis.
For a more detailed analysis of Geoffrey Crossick's report for HEFCE, see Janneke Adema, 'The Monograph Crisis Revisited', on her Open Reflections blog. Geoffrey Crossick has replied at length to 'The Monograph Crisis Revisited' and provided a response to some of my questions in the comment section of Open Reflections.