I'm several pages into the chapter on the job market for scientists. Full credit to Stephan: She tries to make her comparisons as apples-to-apples as possible. For instance, on page 155 she doesn't show data comparing the salary of a new PhD with a new BA/BS recipient. Rather, she compares PhDs 0-9 years out of grad school with BA/BS recipients ages 25-34, when they have a few years of work experience.
A few interesting things so far:
1) Although engineering PhDs generally make more than physical science PhDs, who generally make more than life science PhDs, the early-career income ratios (PhD salary in each category divided by BA/BS salary in each category) pretty much track each other over time. All three types of PhDs can seek employment in multiple sectors, so they all seem to track the overall economy in tandem rather than the fortunes of any one particular sector.
UPDATED: 2) The PhD premium (relative to a BA/BS) is real but volatile and (apparently) declining. She has data through 2006, showing that the PhD premium for an early-career life science PhD was barely 5%. For physical science it was about 25% in 2006, and for engineering it was about 40% in 2006.
The volatility makes sense for a specialized credential attained by small numbers of people. You'd expect it to be risky.
3) The later-career PhD premium is also real and somewhat more stable but still not what it used to be. The ratios also converge, probably because people in every category branch out into more types of jobs (including management) as they progress in their careers. An engineer working in management at a biomedical device firm probably doesn't make much more than a similarly skilled manager who works in the same firm but has a PhD in biology. Either they know how to make their teams succeed or they don't.
4) You're still better off with an MBA.
5) For all the talk of scientists only caring about science, PhD production tracks the unemployment rate several years earlier (people are more likely to stay in school if jobs are hard to get) and the salary prospects for their field several years earlier.
It's almost as though scientists were human beings who responded to resource scarcity and incentives.