I'm of the opinion that the answer depends on how important the subject is to the story. There's a sliding scale. On one end are the subjects that you absolutely need to know to write the story, and on the other end are the fun details that most people who aren't experts or nerds won't even notice you got wrong.
The Sliding Scale of Research
- The book can't be written without an in-depth knowledge of bananas. Assuming the book is fiction, you don't have to get a Ph.D in bananaology to write it, but reading a couple books on the evolution, economy, diseases, and harvesting of bananas will be necessary. You may want to talk to a bananaology or a banana farmer too.
- One of the characters knows an awful lot about bananas without being a bananaologist. Again, reading a book or two is a good idea. Holding a banana tasting party may or may not be going too far. It depends on how much of a connoisseur your character is.
- The book is set somewhere there are banana trees. You occasionally make references to the number of leaves, the amount of fruit, and possibly a secondary or minor character works as a banana picker. You'll need to know the growth cycle of the tree so that it's not producing fruit at the wrong time of year, and you'll need to know what it takes to harvest bananas. A couple magazine articles might be all you need.
- One of your characters eats a banana. You need to know what a banana looks like, and you need to know how it tastes. The simplest solution is to go to a grocery store and buy one, then eat it.
- The book is set in a fantasy world where bananas don't exist, but something very like them does. Again, you need to know the look and feel and taste of a banana, but only so you can mess around with them for your fantasy fruit.
For the previous WIP, I spent a fair bit of time researching electricity. I didn't read physics papers or talk to a physicist, since the book's fantasy and I'm allowed to be not exact, for the sake of the story. I did, however, look into how much electricity it would take to kill someone, and what electrical burns look like. I also looked into energy weapons, space drives, forcefields, hydroponics, and the Great Depression (though admittedly not much on that one). I have online Mandarin and Japanese dictionaries bookmarked, and I spent a couple hours looking at Chinese and Japanese art styles. I still need to find somebody with a Tesla coil, and I need to schedule a meeting with the local police department. That last one is insanely important, but since the police bits aren't finalized, I don't have all my questions yet so am putting it off.
For the current WIP, I've been reading up on gnosticism because it's heavily going to influence the world-building and climax. I'll need to read various mythologies as well, starting with Trickster myths and then heading into anything that has a secondary world. Beyond that, I think pretty much everything is fudgeable because magic isn't real and the settings I've picked are either mundane towns or magical communities. It's modern day, so my characters have cell phones, cars, coffee shops, airplanes, and the like. I may have to look into blade weapons, though. I don't know yet.
What have you researched? For the book you're working on, what do you still need to? What's the most glaring lack of research you've encountered?