Dougpaul, Mad Townie have it right (and I think dave.m's got the gist but his wording is unfortunate): anything written down (in the US since 1979(?) and any Berne convention country) is instantly, automatically copyright by its author (though in the case of a "work for hire", an employer is considered to be the "author" for this purpose).
Title 17 (Copyright Act) Section 102.
There's a "fair use" exemption for scholarship, criticism, parody, etc, but a book consisting entirely of other people's log entries doesn't sound like a strong candidate (just a first impression, not legal advice). Section 107.
On the practical side, though, I think the risk of anybody suing over the copyright in their log entry is pretty low (DEFINITELY not legal advice).
As for websites, "compilation copyright" by the website owner [even if valid] does NOT negate the underlying copyright of the individual authors in their individual works (lawyers call it a "thin copyright", in a different sense). (section 103, section 201c) What purposes the website owner can re-use content for is a matter of permission from the contributors. (Except that the compiler can duplicate the whole compilation, and revised editions thereof. Section 201c.) Note that copyright *transfers* have to be in writing & signed (Section 204), but a *license* to re-use the content is another matter. A well-designed notice ("anything you post to this site I can use in my future book"), if given in advance, should be effective - basic contract law applies. Such a license doesn't actually "negate" the contributor's copyright - the contributor still has the right to dublicate, republish, etc his own comments, and can sell that right.