The Dash achieves this by having one or more Lenses that each are responsible for providing one category of search results for the Dash. The user may search the Lens either through the Dash home screen (called global search) or through the Lens' own page in the Dash by clicking on the Lens' icon on the Lens Bar.
On it's own, a Lens is not very useful. It can guarantee an icon on the Lens Bar and a page in a the Dash but nothing will happen when the user searches it. This is because a Lens does not actually perform any searches itself. Instead, a Lens will have one or more Scopes which are the actual engines that do the searching for it.
This means that it is possible for new Scopes to supplement results of existing Lenses i.e. a Google Docs Scope can supplement the results of the default Zeitgeist Scope in the Files Lens, with results appearing side-by-side. Also, this means it is now possible for one Lens to have many Scopes i.e. the Music Lens can have a Banshee, UPNP and Spotify Scopes providing results to it.
Install Unity Books Lens on ubuntu 11.10 (Oneiric)/11.04(Natty)
Open the terminal and run the following commands
sudo add-apt-repository ppa:davidc3/books-lens
sudo apt-get update
For ubuntu 11.10 users run the following command
sudo apt-get install unity-lens-books
For ubuntu 11.04 users run the following command
sudo apt-get install unity-books-lens