I recently finished "Elephant Time, Mouse Time" that I started last year. Took a little longer to read than I thought. Unlike fiction, you want to read popular nonfiction fairly closely so you don't miss anything, and there's plenty of specialty words even in a very accessible book like this. Not sure when I'll have use of words like "echinoderm" (it's 棘皮動物 for those who want to know) but I'm sure it'll come in handy.
So what's next? For now, nothing. Instead I'm trying to read the newspaper. I select one article from the previous evening paper and read it during my commute in the morning. One article may not sound like much - and the first read-through is fairly quick - but with the heavy kanji use and subject-specific vocabulary the second reading is usually slow going. And if I ever finish a piece before I arrive I have a whole paper worth of other articles to choose from.
So far it does seem to improve my vocabulary more than reading a book. Not surprising, as a book is all about one thing throughout while articles are about wildly different subjects. On the downside, a newspaper is a bit large and bulky to wield in a cramped commuter train. We'll see how it goes.