This book is kind of hard to summarize because it doesn't exactly take a stereotypical view of what a nice guy is.

Basically in the book it says a Nice Guy is fundamentally a guy who doesn't believe they are good enough. They are ashamed of who they are especially expressing their selves sexually so they cope by being nice. One of their main issues is that by being nice they "covertly" establish a "contract" that they therefor deserve certain things in return (sex / relationship / favorable attention etc). And this is one of the fundamental reasons Nice Guys are not seen as good guys because they are fundamentally manipulative.

Also Nice Guys because they don't have real confidence they then don't really put their real selves out there. So since they don't put their real selves out there there is nothing for women to be attracted to, just a facade of nice behavior. Also the nice behavior is out of touch with reality because most nice guys do what they think other people think is nice. They get these beliefs from TV / movies / feminist culture instead of real world which is greasy, raunchy, willing and wholly all over the place.

The reason they don't have confidence though normally stems from a bad childhood. They had parents that didn't treat them with respect, or weren't there enough, or smothered too much and they developed a paradigm of needing validation externally instead of having a core set of friends and family (including themselves) that they fell back on instead. They solved this in child hood by being nice, safe and reserved. They tried to be the opposite of the cruel world, a savior, at the expense of themselves and being safe. Then they continued (unknowingly) this paradigm for interacting with the world in to adult hood.

It says the solution to being a nice guy is to come to an understanding that you and all your flaws are exactly what everyone wants. They want you to be yourself with all your pitfalls, human, and to set boundaries for things you don't like. They want to know what you really like or don't, even if it is perversive (at least in your opinion, regarding sex with women). They don't want you to be perfect. They want you to put yourself first sometimes, take care of yourself, eat right, exercise, take yourself out to do things you like etc.

Also it mentions nice guys don't actually tend to have other guys in their life. It says nice guys should try to have 1 to 3 guys that they go out with (basketball, bowling, etc) that have some of the features a nice guy would think would be in the model type of guy. Also the model for setting boundaries is to set what is required to maintain your self respect / ego. You are already good enough, especially when you are amongst guys and women who are like you. So don't let anyone or anything convince you otherwise.

Basically it says in order to break the belief that is self sabotagingly maintained and protected that they are not good enough, they need to go out in to the world, believe they are good enough, respect themselves and take care of themselves then surrender to the truth with which the world will respond with.