While I haven't written a Dojo book, I did do a technical review of one. David is right, though Dojo is not a bad library, I'd still recommend jQuery as I prefer its approach. Dojo is a bit heavy for my tastes.
But ultimately, it depends upon your own needs and approach you prefer.
Thanks for very helpfull replies. Time to learn new library in our project is very crusial and we are looking for easy to use library. So jQuery will be our obvious choice if its learning curve is less than that of DoJo. I'm also evaluating jMaki, a wraper on various Ajax based libraries. I think it will lot easy to work with wraper classes as compared with scripting everything (yes it loses certain control over widgets and less flexible.) jMaki at time is not providing wideges from jQuery but it is wraping many others. So if we find our required widget jMaki should be OK. I will take a detailed look into jMaki.