Have a search for the free books. You won't get the likes of the big thriller writers such as Lee Child, James Patterson or Harlan Coben's latest, so it's a bit of a lottery, but for the price, download a handful that you like the look of and you'll find most are good. There's nothing to lose.
If there's any classics that you read at school or whenever, that you want to revisit, they're often free, or nearly. An example being Bram Stoker's Dracula and many of Charles Dicken's books (a great observer of Victorian society whose relevance still resonates today). As for thrillers like the above authors, go for the books that are a year or two old and you'll get them for a couple of quid.
Often, newly released books in Kindle format are more expensive than an actual book, for tax reasons I believe. If you like military type reading, something a little different is "Rifles: Six Years With Wellington's Legendary Sharpshooters." It tells the story of the first rifled firearms in the British army from formation through the Peninsula wars. Sort of an earlier British Band Of Brothers. It's around a fiver as I recall but worth it.
The only other things I'd say is get a few books, you aren't restricted by the weight of them, so rather too many than not enough. And have a bit of variety.