I've mentioned before how my Japanese is "just good enough to get me into trouble" — meaning that about half the time I can read right off the page without even looking at a dictionary, and the other half the time I struggle and weep over every single kanji. There's also another level of comprehension that I'm trying to build, the kind you can't always form with a dictionary but is rooted exclusively in current trends.
Consider the term wanpatan (ワンパタン), which I came across almost by accident and only knew the meaning of from a footnote. Given that it's written in katakana, that should be a heavy tipoff that it's an imported word: "one pattern". Meaning boring, or monotonous. (I immediately flashed back to another import, haikara (ハイカラ), "high-collar", meaning someone Westernized but which apparently can also mean "classy in a Western way".)
When I was translating Batten, the term tsukeuma (付け馬) was explained right in the book itself, but I still consulted what third-party material I could find about the meaning. Kodansha's Basic Japanese Idioms had an expanded definition, and it was thanks to Google that I was able to read an excerpt directly from the book itself. (That's encouraged me all the more to buy the thing, so anyone who says this is causing the death of book sales can kindly go stuff it.)
These are things you can't always rely on a guidebook, even an up-to-date or exhaustive one, to keep on top of. They require people on the ground, as it were — folks living in the country in question who stay on top of trends and keep their ears clear as to what words are entering or leaving the vocabulary. It's something I need to find another regular source for, especially now that Mangajin is long gone and the best guidebooks tend to only be revised every decade or so.