Yup, and if you want to "complete the whole set", a couple of other books on lightweight backpacking are "Trail Life: Ray Jardine's Lightweight Backpacking", by Ray Jardine and "The Boomer's Guide to Lightweight Backpacking", by Carol Corbridge.

I don't think of the first two that I mentioned as incredibly "hard core", or at least, they don't necessarily force the reader into a "go crazy over this stuff or leave it completely alone" sort of false choice. I personally would still go with one of the first two that I mentioned, leaving the reader to sort through for themselves what kind of ideas appeal to them and which do not (yet ... or perhaps ever).

To be fair, I haven't read Karen Berger's book in detail (I don't think?), but having been through the others to some degree at least there is of course a lot of redundancy between them. Jardine is renowned for being pretty opinionated, so good to calibrate that and recognize that not everyone considers his personal opinion on various topics the "one and only valid way", but his personal spin can be helpful too.

Still, my suggestion is to pick just one book, and for me it would be the one by Ryan Jordan.