I give away physical books I liked to friends, and those that I didn't like to Books for Amnesty. While you can email someone the .epub, it's not quite the same. A scribbled dedication will persist longer and be more evocative that an email too.
Technical books I tend to keep as ebooks on my desktop where I work anyway.
While I have friends who read, they admit they don't read as much as they used to. So giving books or even games to them often results in an expensive dust collector.
And while I admire the act of giving books away, either to BoA or one's local library, I like to go back to my book every once in a while to recall a specific section that resonated. E-readers allows me to save highlights in the cloud, where I can go back to find particular interesting tidbits and find the fuller section if need be. This is helpful complicated concepts that often require multiple reads over time to sink in.
Technical books I tend to keep as ebooks on my desktop where I work anyway.