Sarah Knight has written an effective parody AND an effective self-help book. The topic of the self-help aspect is prioritization of time/energy/money in line with one's values. Most of the book is devoted to introspection on what aligns with your values and figuring out how to say no to things that don't align with your values but do exhaust your time/energy/money, without being an Asshole (TM). She explores some of the domain of being polite vs. being honest while saying no.
The books weaknesses require little contemplation to identify. Her evaluation function is too present-focused. It is only at the end of the book that she makes any effort to help the reader figure out what they should be doing now so that they don't wind up regretting not doing it later. A lot of duty/obligation stuff is devoted to getting you to avoid this situation, and since she is jettisoning duty/obligation, I think she should have spent more time on this.
But you know? It's her book. And it is fairly humorous.
It's hard to know if this would _help_ any particular reader. I don't know that I was helped by it (but I was vastly entertained, and a little disturbed by how many of the examples it wouldn't even occur to me to feel any guilt about). But if you feel like you are being nibbled to death by small requests difficult to say no to, but which are not advancing you along your spiritual path, hey, can it really hurt? There are at least several giggled in it.