The Igor Pro manual ships in PDF format with Igor Pro. You can access it via Igor's Help menu.
The PDF manual is also included with the Igor Pro demo.
You can download just the Igor Pro manual in one big PDF file here.
You can also view the Igor Pro manual via the web using the links on this page.
(Links between the following PDF files may not work depending on your operating system, browser, and PDF reader software. Links in the actual PDF manual do work.)
Volume I - Getting Started
Volume II - User's Guide Part 1
Volume III - User's Guide Part 2
Volume IV - Programming
Igor Pro 9
Igor XOP Toolkit
Igor NIDAQ Tools MX