Such as amazon and barnes and noble.
I’m sorry, but it is not. For ebook stores of Sony, Amazon, and Barnes and Nobles, they all have DRM so they are not readable among each other. They can only be read on specific devices.
Among these three ebook readers, I’ll suggest the Kindle, because it has free wireless and text-to-speech function. Amazon also provide the best ebook content for the Kindle too. There are some good comparison between them here:
http://www.amazon.com/gp/redirect.html?ie=UTF8&location=http%3A%2F%2Fwww.amazon.com%2Freview%2FR3ULAFVHBGYVQP%3Fie%3DUTF8%26ref%255F%3Dcm%255Fcr%255Frdp%255Fperm&tag=moreinfo-po-20&linkCode=ur2&camp=1789&creative=390957
http://www.speedreaderxreview.com/kindle-2-0-vs-sony-reader-prs-700bc-comparison
Hope it helps 
I’m sorry, but it is not. For ebook stores of Sony, Amazon, and Barnes and Nobles, they all have DRM so they are not readable among each other. They can only be read on specific devices.
Among these three ebook readers, I’ll suggest the Kindle, because it has free wireless and text-to-speech function. Amazon also provide the best ebook content for the Kindle too. There are some good comparison between them here:
http://www.amazon.com/gp/redirect.html?ie=UTF8&location=http%3A%2F%2Fwww.amazon.com%2Freview%2FR3ULAFVHBGYVQP%3Fie%3DUTF8%26ref%255F%3Dcm%255Fcr%255Frdp%255Fperm&tag=moreinfo-po-20&linkCode=ur2&camp=1789&creative=390957
http://www.speedreaderxreview.com/kindle-2-0-vs-sony-reader-prs-700bc-comparison
Hope it helps
References :
http://www.amazon.com/gp/redirect.html?ie=UTF8&location=http%3A%2F%2Fwww.amazon.com%2Fs%3Fie%3DUTF8%26rs%3D172282%26sort%3Dpmrank%26ref%255F%3Dsr%255Fst%26keywords%3Dsony%2520reader%26bbn%3D172282%26qid%3D1256199697%26rh%3Dn%253A172282%252Cn%253A%2521493964%252Ck%253Asony%2520reader%252Cp%255F4%253ASony%252Cp%255F36%253A10000-99999999%26page%3D1&tag=moreinfo-po-20&linkCode=ur2&camp=1789&creative=390957 (Sony Reader)
http://www.amazon.com/gp/product/B0015T963C?ie=UTF8&tag=moreinfo-po-20&linkCode=as2&camp=1789&creative=390957&creativeASIN=B0015T963C (Kindle 2 after price slash)
No, they are not compatible. Each ebook store protects itself so the ebooks can only be read on the devices that are allowed.
If you concern quite a lot about available content for your ebook reader, I’ll suggest you to get the Amazon Kindle instead. It provides the best and cheapest content. Other benefits of the Kindle over Sony Reader include the free wireless internet access, and the text-to-speech function.
Read here to know more:
http://www.amazon.com/gp/redirect.html?ie=UTF8&location=http%3A%2F%2Fwww.amazon.com%2Freview%2FR266PE1Q47CYNH%3Fie%3DUTF8%26ASIN%3DB00154JDAI%26nodeID%3D%26ref%255F%3Dcm%255Fcr%255Fpr%255Fcmt&tag=more_info-20&linkCode=ur2&camp=1789&creative=390957
Hope this helps.
References :