In a standard ePub:
You can not email the highlighted text directly, but you can select the highlighted text, copy, create a note, then paste.
You can then go to the table of contents page and select the notes tab. A the box arrow option at the top right will give you an email option.
The resulting email is quite nice, making sure you have all the info you need to give credit for quotes, or references.
In iBooks Author text books:
Notes are both better and worse. There are more tools dedicated to the management of notes. You can even create study cards from them. The notes icon is right next to the table of contents icon in the toolbar. Tap in the middle of the screen to see it.
But; you can not copy text and paste it into a note (at least not in the few books I've tried); and like ePubs only the notes you type (not highlighted text) can be emailed.
The resulting email looks similar.