1. A user's guide and a reference manual for desktop publishing application (framemaker, word, openoffice whatever..)
2. user's guide and ref-manual for your source code management system
3. scripting languages - I have books and find them more useful than the web. I use awk, sed, flex&bison, Perl, Csh, bash, Tcl and Expect for various automation purposes. Like me, if you don't use them often enough, you are likely to forget the syntax and start over every time you need to write a script. O'reilly books are great and cheap. Buy one set for every cube or every aisle in your office.
4. What you also want in your locked cabinet is a digital camera. A computer sucks for illustration purposes, although it is great for presentation. So you write down and draw on paper or white-board and you take pictures of it and keep it around. Handy when you file for patents or for inclusion in documents for quick turnaround.
5. For increased productivity, know how to use your company phone, instant messaging and other conferencing facilities. The reference cards are probably lying around near the admin's office.
The rest depends on your actual stream of work and area of expertise.
I find the Frame Maker user's guide and reference manual absolutely essential for creating neat documents with all tricks for easy reading. As a project lead, you are likely to need all tips/tricks to create your clean and professional templates for specifications. I think every incoming project member must be handed a user's guide and reference manual for the documentation software in use. I am quite sick of seeing badly written documents with not enough attention paid to it.
Another book that is of immense use is a reference manual for your source code management system (Clearcase, Subversion, Perforce, CVS whatever...). I actually use a couple of them. You don't want to have to hunt for your buildmeister all the time. Every programmer eventually needs to develop strong skills on managing one or more the source code management systems, over time.
I use two monitors... the left one is dedicated strictly for writing documentation and for reading online documents. The right one is for programming (well... and for web-browsing and reading news).