Annoying pedantic note: I think it's "Wood".
Um, yeah, it's a hard-to-describe book which includes "generational saga/mystery", historical fiction, mythology, and, extremely interestingly to me, science. I liked the rational exploratory angle the book took in its making mythology and the collective unconscious and history/myth concrete in the symbol/device of the Wood. It eventually breaks the bounds of rationality, of course, but it takes that approach and, outside the wood - well outside - it's still (in the modern sections of the story) the modern real world. I can't remember the plot too clearly but I think there were both internal threats (in the sense of the protagonist becoming involved with what was going on inside) but also the traditional "incursion" motif as the Wood threatened to break into the "real world" - am I recalling that correctly? (It's been eons since I read it.) Anyway. yeah, it's a book most any fantasy or SFF fan would love, I'd think, and should even interest SF fans. It feels fresh - it deals with very old concepts in a very new way - it doesn't feel like a rehash of the usual tropes even though it's not at all divorced from them. So I agree with the "unlike any book" thing.
For myself, I didn't utterly love it because it's not directly exactly my kind of thing (and I even stupidly got rid of my copy after keeping it for years in an overzealous minimalizing binge) but I certainly enjoyed it and think it's "recommendable". I will say, though, that as fresh as it was, it seemed to trap the author and cease being fresh, in that
Lavondyss was a less good sequel - still fine, but not necessary, and I believe I read another sequel (there were several) that I didn't really like. (I think it was that, and getting rid of those, that sort of led to a negative drag that pulled
Mythago Wood, itself, out of my collection when it shouldn't have.)