If you're sick of reading book reviews, I'll give you the short version. If you are a project manager, designer, developer, entrepreneur or business analyst who is involved in creating a (web) application read this book. It's awesome.
Now for the longer review: although the guys from 37 Signals just published Rework, I still had to read their previous one called Getting Real. They describe their process of creating popular applications like Basecamp, Campfire and many others. I got great value out of it even though I'm currently not developing my own products. It has great advice on subjects like how to deal with clients and users.
For example, scope creep is something everyone involved in a project should really look out for. A web application is never finished and clients will hold the release until the perfect product has been created. Instead they state that you should get your application out into the world as fast as possible. You will instantly see what works and what doesn't. And no unnecessary features will be introduced just because the client thinks it will be a big hit. This is based on the "Release early, Release often" idea and that's also what I try to do at Two to Tango when developing software projects. On top of that the book advices not to fix every bug, especially layout bugs. Don't worry about that button that is 2 pixels out of place in Internet Explorer 6. Just release your application and let the magic happen.
About users they say the following: don't listen to your users. Don't implement every little feature that they request. Instead have a clear vision about where you want to go with the product and stick to it. If you listen too much to your users, you'll get an application that serves it all. I see it as an application without a spine. They advice to build an application for yourself not for your users.
I just described two points that I took away from he book but it is filled with beauties like this. So just go read it now, you won't regret it! You can even read it for free online. And read Rework also while you're at it, I'm pretty sure it'll be worth it.