Ah the cloud... It is everywhere, right? But what has it done for you lately? Well how about acting like a big hard disk in the sky that is transparently accessible across all your devices: PC, Mac, iPhone, iPad? Dropbox is the answer. Install the client on your PC and Mac and download the app for your iPhone and iPad and you're away. It looks like a normal drive to your computer and you can edit files directly from it. The client keeps a local copy and synchronizes it to the cloud so your file is the same - everywhere. It is probably the best way to get content on your iPhone or iPad as well.
For a product that is a whole lot more than a book reader, the iPad offers the most choice when it comes to reading ebooks because it has both the Amazon Kindle and Barnes and Noble Nook eReader apps (Nook app is currently still an iPhone app), Apple's own iBooks app and has first class support for PDF (which the Kindle certainly can't claim to have.) So if you want technical books, like computing ones, here are some choices:
Amazon Kindle Store. 18,098 titles in Computers & Internet. This includes many self-published and items of dubious value.
Apple iBooks. Embarrassingly, there are no technical books here (not even one on Cocoa programming or Objective C.)
There's another service you may not be familiar with:Safari Books Online. With over 11,132 items, all of which are either technical or business related, this is a really good resource. The catch is that it is subscription only and renders many book pages as images on the iPad (you need to use the mobile version of the site, or it will try to use Flash.) Being an online only service, it requires you to be connected to the Internet and is accessed via the web browser. The cheapest subscription is about $23/month and that gets you up to 10 titles at a time.