The book I wish I had alongside my dictionary, my thesaurus, my dictionary of etymology, and so forth, would be this: a book that lets you look up words that parallel other words. An Ariadne, to lead you out of the labyrinth of ... oh, screw the metaphor.
For example, to render a particular sentence pleasingly parallel, I just now needed a word that means "to make taller" -- a word that is to height as "widen" is to width. The word "heighten" doesn't fit; it means to raise something higher, not to make something taller, and anyway it's mostly used figuratively. Nor is "grow" quite le mot juste, since it doesn't refer to tallness specifically. The word I want is "tallen," only there's no such word. (I ended up writing "grow vertically" -- ick.) To add insult to injury, there are at least two similar words for "widen" -- the other being "broaden" -- and no "tallen" at all(en).
So I want a book that, when I looked up "widen," would tell me the corresponding word for tallness -- or at least reassure me that no such word exists, so I could stop wasting my time looking for one and skip to the step where I either coin it or struggle on without. Alternatively, or in addition, if I looked up "tallen," it would tell me that that's not a word, but the word you want is ... well, whatever it is.
A thesaurus sometimes solves these problems, if you happen to think of the right related word to look up, but not always. In this case, I tried thesaurizing (is that a word?) words such as "expand," "grow," and "extend," until I gave up. Was I insufficiently creative in seeking synonyms? Not in a properly lateral frame of mind? Or does the word I'm looking for simply not exist? No thesaurus can tell me.
While it's a snap to find books of obscure words, they're invariably mere entertainment for logophiles and sesquipedalians; I don't find the kind of serious reference work I want. Maybe this is because the kind of people who might write one can't take the word "tallen" seriously enough to invest the effort. Occasionally, in specific cases (such as this one, which I ran into recently), good old Google can scratch your itch. Or you might find relief in a reverse dictionary. (Look under "r," har har.) Other times, you're just doomed to wallow in ... um ... what's the word I'm looking for?