I see you already know Niven's Ringworld series, so make sure you also read his Dream Park series (4th book in series released just last month). They're in kindle.
For fun military sci-fi, try Jerry Pournelle's Falkenberg stories. Both Niven and Pournelle are actively working on getting all of their works put on Kindle.
For fantasy, try Raymond Feist's Magician - Apprentice and Magician - Master. Those are the first 2 books in a lengthy multi-branched series, but you can read those two and be satisfied. The next few books in the series are good too, but the last half-dozen or so aren't nearly as well written.
If you want to grow up to be a girl someday, try Anne McCafferey's Dragonsong, dragonsinger, and dragondrums trillogy. Those are fun to read but a bit juvenile. She develops the world a lot more in other books based on the same planet, but those can be read by themselves and are quick and fun, if a bit girly and juvenile. The same goes for McCaffery's Crystal Singer trillogy - girly and juvenile, but I thought the premise was interesting enough to be a fun read especially if you can find the book cheap.
They're way overpriced on kindle right now, but the first couple of "Spellsinger" books by Alan Dean Foster were fun too. Get them for $.99 in used paperback instead of spending $9.99 on kindle...
Almost anything by Robert Heinlein, including his juveniles, are worth reading. His last few books went weird but his early stuff up through "The number of the beast" are great.
For non-fiction, The Reagan Diaries are fascinating.
If you want to know why the Iraq and Afghanistan wars went stupid, get "The Seven Pillars of Wisdom" by T.E. Lawrence. You'll get the picture in the first couple of chapters and then you'll continue reading just because it's like watching a 30 car NASCAR wreck in slow motion but with CNN current-events subtitles added by your own imagination.
I second the recommendation to try the Baen free library. The Honor Harrington series could have been girly and juvenile... but they aren't. Recommended. Pournelle has a couple of books in the Baen free library, and they're both well worth the 30 seconds it takes to stick them on your kindle.