Having just cut my teeth on OSG, I've found two *very* handy references: the Quick Start Guide (which is freely available) and this book. The book could have used much better copy editing, but it provides a solid introduction to using OSG and I've found it to be a handy reference. I went through it chapter by chapter, even making myself do the stuff that I figured I wouldn't need, and was surprised as I continued my own project to find that much of what I thought was not relevant to me actually was. Suffice it to say, the Beginner's Guide covers OSG pretty well, though I'd have liked it to go more in depth on certain topics.
If you're an experienced programmer, you may find the reading a bit distracted by topics which may seem elementary, but those who aren't thoroughly experienced in C++ or scene graph technology may find those diversions helpful. The Quick Start Guide, in contrast, is a bit more technical and harder to understand if you're not already well-versed in these areas.
In any case, I can imagine a better effort for a beginner's guide, but it remains a useful introduction and handy reference as I continue to work in OSG. That said, I recommend the book to anyone who's starting OSG and wants to be eased in to using the software.