that's not an o'reilly book, but again i have heard a lot of good things about the ...in 24 hours books (mainly from X11).
I haven't bought any unix books myself (although i have about ten full lever arch binders full of printouts of essays/tutorials/entire books) but i used to work in an academic bookshop a couple of years ago and got to have a good look inside a lot of these computer books. I would say that
at a glance the ones that look most solid are the in 24 hours ones and the o'reilly ones. There was also another line that looked really good, but can't remember what it was right now (it wasn't the '...for dummies' or 'idiot's guide...' by the way, although they are excellent books for getting totally started from nothing in a subject).
a book i think is very good (in a general capacity) is "Unix unleashed" which is fully available online in at least two editions: "
the System Administrator's Edition and
the Internet Edition. Some of the chapters are the same, but some are unique to one version of the book or the other. If at some point these links stop working, just search google for "unix unleashed" because it is mirrored in plenty of places.