1) Dojo is a complete Ajax toolkit which can be used to do anything from adding a nice-looking calendar widget to a page, to create a dynamic fat client in a web page using 2d graphics, layout managers, type-ahead comboboxes, trees and sortable, skinnable datagrids.
Advantages: A comprehensive, smart and opinionated toolkit which guide you towards better modularization of client-side logic. Disadvantages: The number of features are large and can be hard to grasp, also dojo.data, the data abstraction is a bit complex.
Dojo is very secure indeed. A lo of effort has been put into securing cross-domain access, hardening JSON data transfers, and other things.
7) Heavens, no! And no book does, that is in print today. As a matter of fact, my book is the only one to cover certain interesting layout manages and widgets in the dojox (experimental) branch. But a truly *full* book would easily go over 1000 pages and take a while to write.