There's a lot of talk of ecosystems with modern smartphones and the war raging around where you buy your content and where you consume it. In an ideal world you're free to move your content around and do with it what you want: it is, after all, your content.

Buying books online has never been easier and Amazon has put the tools at your disposal to do almost everything you want. Yes, it uses its own protected format, but it also gives you the apps to view that content on just about any device you like.

Kindle for Android

Google Play

The Amazon Kindle, in all its different guises, is one of the most popular reading devices around. You just have to step on to any train and you'll see one. It's also a device that refuses to be pushed into the shade by tablets.

There's a good reason for that: the Kindle is affordable, it's easy to use and the content that Amazon offers is often competitively priced against other online retailers, and almost always beats the print pricing.

However, you don't have to buy the hardware to benefit from Amazon's pricing on Kindle books as you can access them through any smartphone. Here we're looking at the Android version of this app, and although it isn't new, it's certainly one of our favourites.

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HTC One (left), Nexus 10 (right)

Sign in with your Amazon account and you'll get access to those Kindle books you've purchased in the past. Rather than downloading them all, they sit in the cloud, waiting for you to retrieve them. There's no syncing from your computer, no moving of files or worries about format, it all just works.

But this isn't just about access to books, it's about making the reading process seamless across all devices. That's where Whispersync comes in. It's a fancy name for syncing, but it means that as you read, your progress is logged, so instantly accessible elsewhere. No need to remember where you got to.

You can also drop bookmarks manually, as well as make notes, perhaps if there's something of interest that you want to come back to later. When you pick up a Kindle device, or something with the Kindle app, it will ask you if you want to move to the last read location, which is really handy.

That means you can be reading on a Kindle Paperwhite in bed, for example, and then read another chapter while waiting for the train, or in the park at work and you'll never lose your place.

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HTC One (left), Nexus 10 (right)

Kindle for Android is optimised for tablets, so rather than just getting an enlarged version of the app on a bigger screen, it makes better use of space on a larger display. You get a four-column grid rather than two, and in reading there's a more substantial top navigation bar and it doesn't use overlays like the smartphone app.

There are a range of reading view options on offer too, so you can change the margins, spacing, font size and the screen colour, selecting from white, black and sepia. This enables you to get Kindle for Android to whatever your situation: white text on a black background is better for reading in the dark, for example.

Beyond all these key functions though, what the Kindle for Android app means is that you can buy from Amazon without having to first buy one of its devices. Of course, if you already own one, then you win too, as all these things work together.

READ: Amazon Kindle Paperwhite review

Amazon isn't the only company to offer these services: Kobo has a rival offering, but Amazon seems to lead the way with pricing and it's certainly Kindles that we see in the hands of most people reading.

Kindle for Amazon is an important app. We've used it since launch and it's a great example of how a content provider can expand access simply and easily.