Hi Thomas,
if you'd be living and working in Austria the answer would be: definitively try to get the Doctor grade.
In our smaller neighbor country, having a title is literally worth €...
For Germany it is still a factor when it comes to initial job entry grading for fixed employments. The PhD will typically get you more money or at least a better negotiation position.
However, depending on what kind of job you want to do, the actual effort to gain a PhD/Doctor grade might not balance the outcome.
A MBA or some additional project management or IT certification might serve you better in terms of RoI.
As you seem to freelance or at least to work on a project basis I tend to say: the Dr. won't give you any more money. In case you're the absolute guru/super-hero in your field the title likely won't make you more interesting to your market.
However, if you're interested in the research and the scientific work, well then maybe there's more in it to you then just a pay raise.
Having just finished a SAP Press book myself, I might add: that's also a way to fill your time (lots of it) and to "research" and publish. You don't get a title, but a mug shot on the cover if your book gets printed. 
- Lars