I saw a recommendation of a book by a friend on Facebook, with a link to the Kindle edition. I clicked through to that, and paid for it using one click, confirming that I wanted it sent to my Kindle. I picked up my Kindle and switched it out of "sleep" mode". By the time I'd done that, the book had been downloaded, and I was able to start reading.
The Kindle itself is pretty small and light - with the case we bought separately for it, it's the size of a small paperback. It stores up to 3500 books. In any case, you don't have to keep books on the device. Once they have been bought, they can be re-downloaded to any of your registered Kindle devices - my Android phones also have a Kindle reader on them. There didn't seem to be much point in putting the PC Kindle reader on my computer, but it's an option. We have lots of books in the house, though less than 3500, I think - the thought of being able to extend my library without needing more shelves is wonderful.