Saundby,
Non-science fiction? Hmmm...
- The Flashman novels: They show you how to insert you players into momentous events. They'll also show you how history most likely happened. Flash Harry, IMHO, is one of the most realistic characters in fiction. (The author, George MacDonald Fraser, died somewhat recently so we'll never get the Flashman in the American Civil War book. That may have been planned because IIRC Fraser had painted himself into quite a corner regarding Harry's presence at certain events.)
- The Cruel Sea: By far the best novel about what it's like to serve in a navy during wartime. Imagine boredom mixed with horror mixed with fatigue.
- The Travis McGee series: John MacDonald wrote them and each title includes colors. I used the books for the plots McGee becomes involved in. He acts as a "salvage" expert. He finds money people have "lost" and cannot recover for certain reasons and then splits what money he can recover with the original owner. He's not exactly a criminal, but he's not enforcing the law either. He's a nice example of how a group of PCs can work "gray".
- Any history book about a period that is new to your players: Because historical events happened, no one can splutter about plausibility if you plunder them for adventure ideas even when the events themselves were pretty implausible. How history minded your group is will determine just how easily you'll be able to recycle ideas. My groups were nearly all wargamers and ex-military so I had to plunder events and ideas from various historical "margins" because they were familiar with so much of the rest.
That's all from the top of my pointy head.
Regards,
Bill