I can second Kindle as an option (I have used an older model and a paper white) - for reading novels and other easily rendered text-only fare, it is a great product. The ability to function in direct sunlight as well as darkness without destroying your eyes is a huge advantage over other options.
The Kindle app is available on other platforms too though (Android, iOS, desktop OS) and I have found it extremely useful to have a larger format, color capable reader for a couple of use cases:
- Graphic Novels - if you like the format, then you will need more than e-ink
- Technical reading - diagrams, code samples, text that reacts poorly to being broken up all benefit from the larger format option
I have also used Kindle on my phone (it syncs to the furthest read location on multiple devices) for those times that you forget your reader/tablet or are unexpectedly bored.
Hence my recommendation, if you can afford it, is a multi-device approach for maximum satisfaction. Whether you go with Kindle or not, make sure that your chosen app and format are usable with several types of devices (syncing is so useful too).
For travel, the phone is probably an easy choice (as long as your screen is reasonably sized). The tablet is a personal call - you can replace it with a laptop, but I find that awkward in restricted space (planes, trains) - decent tablets can also be loaded up with films/games etc. for the times your onboard entertainment options are poor. Personally, I leave the tablet at home for travel unless I know that I have a long flight where I need to do tech reading or I know the entertainment options are going to be horrible.