The dip is a very short book, less than 80 pages, that aims to answer a single question: whether you should quit or whether you should stick to something (just like it says on the cover!). It could apply to a project, a market, a relationship, a career decision...
The book starts strongly on giving a sense that "it's ok to quit", which I personally found a bit overstated, but perhaps this is because I realised a while ago that sometime it's time to cut your losses and accept a change of strategy. Later on the author lists some reasons why you should quit, and others why you shouldn't (e.g. you shouldn't quit because you're scared -- that's not much of a strategic decision).
According to the book, your main goal or drive should be to "become the best in the world", and to keep that in mind while making your decision: that is, if you can't become the best in the world, find another problem to solve. I'd say for some of the questions you might be thinking about, it's not always the appropriate answer/goal but it doesn't make the book any less relevant.
Overall I enjoyed and appreciate the description of what is essentially a framework for making a specific kind of difficult decisions. I found the end of the book clearer and more helpful in deciding whether it's the right time to abandon an effort. The first 2/3rd really aim to hammer home why quitting is ok (sometimes) and why you should consider it, or even consider considering it.