I'm reading a chapter on jQuery in a ASP.NET book. To be honest I'm finding it quite boring.
How important is it to learn jQuery? And do many ASP.NET developers use it for their pages?
It is not at all important to learn jQuery with ASP.Net. Many people find it useful, but the two are not even remotely related. It is entirely possible to build a full-featured, useful website with ASP.Net and not ever once touch jQuery. It is a common framework, and quite powerful if you have need of its capabilities, but it has nothing to do with .Net.
There are a lot of people on this site who are manic about jQuery in that they believe every site/page should have jQuery built in and used when, in truth, it's just a framework. If you have need of its capabilities, then it's absolutely recommended because it's quick, easy and understandable. If you aren't making use of it on a regular basis, then you're just wasting overhead on loading a framework that you don't need.
That being said, I would still recommend that you tough it out and make it through the chapter because you may find that jQuery holds framework functionality that you were unaware of but would find useful. It's not necessary, but i could be very, very helpful.
Edit: I forgot to answer your last question. Yes, it is a very popular framework that is so common that Microsoft included it in the ASP.Net MVC distributions as a standard.
The reason for this is that the DOM interfaces are really rather poorly thought out and often buggy. I have heard it said that every major interface in the DOM has at least one major bug in one browser. By using JQuery you create a layer over the DOM that is consistent and well tested.
It is possible that the chapter in that book is just poorly written. There are a number of very good JQuery resources around the net, starting with the JQuery web site.
From experience I never thought I would have to learn jQuery for ASP.Net development. But over time I have been proven wrong. This has more to do with just general web development as opposed to .Net vs jQuery development. There is so much focus on jQuery and the use of it that eventually you come upon some piece of functionality that you cannot easily reproduce in .Net. You will find knowing the basics of how jQuery works and how to effectively integrate it with your apps will save you a lot of time down the road. And I will echo what others have said making if you make the move to the MVC framework in the future jQuery integration is one things you will need to know to effectively use it.