Dear Patrick,
The Hathi Trust puzzles me. It's a major consortium of USA libraries that has been busy digitizing books on a grand scale. But almost everything they have done is not available for consultation. This is especially true if you live outside the USA. Personally, I have given up trying to use the Hathi Trust's materials, because I repeatedly see desirable items in their collection but I can't get at them. Maybe it works better if you are reaching them from a USA university network?
I've corresponded briefly with the folks at HT, and they said their hands are tied by copyright issues. However, many items I've tried to view were long out of copyright, so that's not quite correct. I think perhaps they have a blanket bar on lots of materials because they don't have the staff to pick through everything and decide what can be released. But other consortia manage this problem simply by using a date of publication and making reasonable assumptions about the lifetime of the author.
I also don't understand the relationship between the digitizing that HT does, and the work of Google Books, DLI, Microsoft and Archive.org. Are HT not duplicating work done already, elsewhere?
I have not had time to dig into these questions, so if somone here knows more about HT's policies and goals, I'd be glad to learn more. HT looks like a valuable resource. It would be good for our field if we could actually use it.
Best wishes,
Dominik