My bookshelves hold a wide assortment of reference books, from several different style manuals to ancient dictionaries and thesauri. As for the latter, even though I keep the print versions (probably for nostalgia's sake) it's just so much easier to go to Dictionary.com, or to right-click on my mouse for a list of synonyms.
One of my favorite style manuals is "Strunk & White's Elements of Style." This handy little book has been around for decades. It has such helpful chapters as "Elementary Rules of Usage," and "Words and Expressions Commonly Misused."
A side note here: Recently I noticed that the newer version of Strunk & White doesn't list "irregardless" as a word commonly misused, while my old version does. Though I'd always been taught that "irregardless" is not a word, apparently it's becoming acceptable through frequent use. This is what Grammar Girl has to say about it:
There are other, bigger style manuals, but Strunk & White is a good place to start.
The Associated Press Stylebook is the source to use if you're interested in writing for newspapers or news magazines.
Then there's "The Chicago Manual of Style," which is a wealth of information, but at nearly a thousand pages the print version could cause a hernia just to pick up. An online version is available for a yearly subscription fee, but it's so convoluted that I find it frustrating to navigate. It's primarily for book authors, so if you have the patience to shovel your way through it, it can be well worth the effort.
And for all of you writers out there, the king of reference books--the bible--is "The Writer's Market," which is updated yearly and lists book publishers, small presses, magazines (by category) literary agents, and even seminars and contests. Once you're serious about sending your work out in the hope of publication, this book will be invaluable. It's available in hardcopy as well as online (for a paid subscription) at www.writersmarket.com. Last time I looked, the Spencer Library had the 2007 version, which, though outdated, is still full of useful information until you're ready to buy your own copy.
"Writing is the hardest way of earning a living, with the possible exception of wrestling alligators." William Saroyan