Tell Me About Your Indispensable Programming Reference Books

At the suggestion of some colleagues that it’s a good instructional volume, and having heard that it was a fairly indispensable reference to have on hand, I purchased and am currently working my way through Mastering Regular Expressions.

(Let’s just say there was a moment this week where this stuff made sense, and I really want to hold on to that as much as I can.)

Is there a book or two that you use on a regular basis or that made something with which you had trouble suddenly make sense? Online resources are OK, too, but I’m looking for things that address stuff like programming concepts, tools, or languages. Tell me about those books or resources in the comments.