Like most people (I assume), I do a lot of research online. But for some things, I still need (want) a book I can flip through. So, I keep a collection of those on my bookshelf.
Here are some of my favorites.
1. A naming book (I have seven or eight of these, but From Aaron to Zoe is my favorite). I also, yes, rely on the internet and on a personal list I keep in a notebook where I can reference which names I've already used and when/where they were used.
2. A couple of visual dictionaries. These are useful when you know what the whole object is called, but you don't know the individual parts. I recently found a science visual dictionary, but I'm not sure when or if I'll be using that.
3. Several usage manuals. I have more, but The Chicago Manual of Style, The Merriam Webster Dictionary of English Usage and Words into Type are three of my favorites. These are especially helpful when I'm editing and get into one of those head scratching moments when I've forgotten the rule that applies. For the record, I've also occasionally subscribed to The Chicago Manual of Style's online presence (where questions can be asked or where a person can access questions others have asked), but that costs money that must be paid annually, so usually I just rely on the book.
4. The book that is standing up is one I found at an estate sale the other day. It's by Rodale of thesaurus fame, and it's an interesting addition. I can look up a word and find other words that might be used to describe it (such as Honey, described as pure; limpid; thick; flavored; fine; fermented; amber; sweet and nectareous. I doubt I'll be using the word nectareous any time in the foreseeable future, but this is still a useful tool).
I also have books listing flowers, trees, birds, and buildings, among other topics.
What can I say? The internet is great, but sometimes a writer just needs a good book to help her (or him) along.
Have a great day!