Disclaimer The opinions expressed herein are my own personal opinions and do not represent my employer's view in anyway.
A while back it was impossible to even ask this question because there were no SharePoint v3 books out and then for almost a month there was only one. But now there's a choice so it's decision time.
(All the books are listed here http://www.asaris.de/sites/walsh/Lists/WSSv3%20FAQ/V%20Books.aspx and all the one's mentioned before have icons on the right-hand-side of that page [leading to Amazon US - the items in the center also have links to Amazon UK, Amazon Germany and Amazon France]
To my mind there's only one book on anybody's must list and that is the Administrator's Companion. I was always referring to the equivalent v2 book - for instance even though I'd done tens of installations of WSS 2.0 and SPS 2003 I still referred to that book and the Microsoft installation documentation every time I did a production installation (and the book was where I kept my scribbled notes on those installations).
The rest of the books that are out make for a more interesting discussion. Both the Todd Bleeker and the Scot Hillier book are for developers and yet at the same time they contain enough useful information to be of great value for Admins who have no intention of doing any developing.
Todd's book "Developer's Guide to the Windows SharePoint Services v3 Platform" contains a marvellous set of chapters about what is new in SharePoint v3 that will be useful to anyone coming to the v3 products from the v2 SharePoint products without being heavily involved in the v3 betas (and probably to those that were as well).
Scot's book "Microsoft SharePoint 2007: Building Office 2007 Solutions with C#" has apparently (the word from a source I trust on this) the best chapter on how to set up a development (virtual machine) environment that's out there. This doesn't surprise me. I found the first chapter in Scot's second v2 book (SPS 2003 and WSS Books - Advanced SharePoint Services Solutions) to be so useful that I used to say (and think) that it was worth the cost of the whole book by itself. I suspect the same is the same with this chapter in this v3 book (although personally I'm waiting for the equivalent VB book to come out before buying it).
The final pure SharePoint book that's out is the User's Guide "SharePoint 2007 User's Guide: Learning Microsoft’s Collaboration and Productivity Platform"). This one I've not seen so take the following comments with a pinch of salt. If you want a User's Guide at the moment (i.e. presumably a general SharePoint v3 book that doesn't go into the details of the Administrator's Companion) by all means consider it as it's for now the only one out there than covers this area. I remember not being over-impressed by the equivalent v2 book (capable rather than full of new knowledge) but that might have been because it came out relatively late and I by that time already knew quite a lot about the v2 product(s). As I say, treat my comment above about this book with a piece of salt.
As for the final book on that list of icons, that's a book about workflow. Workflow is a key area in the v3 SharePoint products and no doubt if you have the time to study this book and if workflow projects are your main area of SharePoint activity then this book (Workflow in the 2007 Microsoft Office System) is certainly one you should seriously consider. [Note added-in: However you should have a look at the the book first if possible. The writing style didn't appeal at all to the second person who commented this blog item - however it did appeal to someone else who commented!.]
I'll ignore here the final two icons which are for a couple of DVD courses (cost $299 and $399 respectively). I suspect that none of you blog readers are going to fork out that amount of your own money. Also all I can say about them is that the same "writer" did the same two courses in the v2 timeframe as well, so presumably they sold well enough (and therefore were good enough) to make this second v3 series possible.
So there you have it. The answer to which book to buy seems to be "all of them". As always it depends who and what you are buying them for.
Every Admin: Administrator's Companion
Starting with SharePoint: User's Guide
Development: the Todd Bleeker and Scot Hillier books
What's in v3 if you know v2: the Todd Bleeker book
Workflow: the workflow book (!) [but note the question mark in the text above]
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