I think the Kindle Fire is a bit 'jack of all trades, master of none' as in it's a pretty crap tablet that's not great as an ereader either. But it might be useful as a second device in case something happens to your phone, but I usually plan for that by having printouts of key documents (boarding passes, accommodation details, travel insurance etc).
But if you just want something to read, even the cheapest Kindle now has a light or the Paperwhite version is waterproof which is good if you read in the bath, at the spa, or on the beach and you want to be able to rinse the sand off.
I've looked at the more expensive ones with extra features and not thought them worth the extra. To me, the value is in the simplicity and the more basic ones being good at their main purpose, reading. Plus the long battery life.
If you don't want to spend a lot, you could look for a second hand one on ebay or wait for one of Amazon's regular 'deal days'.
I always get books on 99 p deals, so the cost of books isn't really a consideration for me.