I've been told that publishers estimate that one third of books bought are never read and a further third are never finished. Pierre Bayard, in his book "How to Talk about Books You Haven't Read" (you'll find it at BookBrowse) gives numerous examples of books people talk about without reading or without reading fully - and actually suggests that this is not only acceptable but that it's the only way for sophisticated readers to "read" all they want to.
For me, a book is more about the topics that it explores than the content of the book itself. Thus, there are a great many non-fiction books that I've read in part, not in full - picking out the bits that interest me and skimming over chapters that are not relevant to me at that point in time. I would not consider myself having done the book or myself a disservice in doing this - I've just taken from the book what interests me and moved on.
So, I would say that the remarkable thing about your club is that nobody has previously not completed a book - or at least admitted to not doing so! Perhaps this is because you haven't read a lot of nonfiction until now?
In fact, the boffins who study how people read will tell you that nobody reads every single word of a book - we all, to some extent, skip when reading - it might just be a sentence or two of description here or there and it could be entirely unconscious, but we all do it to some degree. Obviously, the amount of "not reading" you're talking about here is much more than just a few words here and there, but the principle still applies - thus, you could argue, if it's okay to skip a few sentences, why not a few pages? If okay to skip a few pages, why not a few chapters?
So, in short, so long as most of you have read some of it, I think the conversation should run perfectly happily, and maybe you'll want to spend a bit of time discussing what elements people found to be heavy going, and whether as a club you're happy tackling books like this every now and then that you're not going to necessarily read cover to cover.
Davina - BookBrowse editor