The Tip: If your mouse has thumb buttons, they are usually mapped (by default) to "back" and "forward" in IE. Change those to PgUp and PgDown, and give it a test drive in Visual Studio. I never noticed how much of my development time was being spent on paging... much better now!
The Tool: Resharper is without a doubt the best 99$ I ever spent on a dev add-in. I find that I don't use the advanced refactoring tools very often, but the simple things such as renaming members and converting variables to properties is very handy. My favorite features are (by far) the many small improvements to the VS.NET environment such as quick-navigation tools and improved intellisense. There are many similar add-ins - some of them freeware - but from what I have seen so far Resharper seems to have the best combination of features, usability and general polish.
The Book: Microsoft Windows Sharepoint Services, Inside Out, Jim Buyens, Microsoft Press. I'm not a big fan of the Inside Out series, but this book is awesome. It covers design, installation, administration, customization and custom development of WSS, and somehow manages to stay readable from the basic concepts through to the advanced minutia in each topic. I usually get frustrated very quickly with non-electronic documentation (bookmarks didn't originate with the internet - who knew?) but this book is one which I actually plan to read cover-to-cover. At 700+ pages it might take me a while, but I'll post a more detailed review when I'm done.