Both their books are quick reads and worth the couple hours you would invest. That said, you likely aren't going to come away from either with a profound new understanding of anything.
You could wrap both up into a couple bullet points.
I file everything by 37signals under the "potentially quite helpful, but take it with a handful of salt" category, the same place I put Seth Godin.
At times they have some really lucid and sensible insights but a lot of the time their cocky attitudes make their thoughts unpalatable.
You'll take the bad with the good, but are their books worth reading? Emphatically, yes.
I personally think they are well written books, with great advice for entrepreneurs.
Different things work for different people. It always pays to listen to and understand the perspective of bright people.
I personally believe, different styles of management are needed when things are going well and when things are going awry. I also believe that a lot changes when companies grow beyond a certain point.
So I would treat these as some very good ideas for startups, yet still keep an open mind while running your own business. The advice should not be treated as gospel, as it is based on the the experiences of some really bright people, in the right place at the right time. A lot of the advice is not applicable for large companies.
It would be cliche to say Rework "changed the way I think about sitting down at my desk to bang out projects"... but it kinda did.
You can go back and read the lessons any time (they are only 1-2 pages). They are terse, which might put some people off who like long winded explanations, but effective. And last but not least it leaves you with the understanding that a few guys with solid skills, good ideas, and a penchant for not having any bullshit - like bloat, bureaucracy, or super strict rules - in the workplace have built a successful company that they can be happy to work at every day and is the envy of many in the web app world.
I would also suggest Made to Stick by the Heath brothers. It isn't about work processes or business building but about how to create messages that people remember. If you have an app that is really valuable to people but you can't tell them how it will help them your less likely to see any real success.
I think they are great insights into lean startups. Not that this would work for you, but it's nice to have another point of view of the entreprenurial process, not only the "get-the-money-and-run" approach.
If you've every worked where you felt you were in a "Dilbert" comic strip and wondered is this the way it has to be, read a book by 37Signals.
The concepts are very general. They get to the point and spare the reader from boring 5 page anecdotes (Some writers just don't know how to tell a story.) that are 'suppose' to add clarification and make you feel all good inside.
Why dont you download them through a Hotfile123 search and read them for yourself. I read rework and found it to be highly overrated.