I find Paula's work interesting and she does get many things right, but not for 100% of the time. I've found several pretty glaring errors in her work. If you want to look at cosmetic ingredients, her work is certainly an intriguing starting point, but I recommend checking from at least one additional independent source.
It's also very difficult to sit on the fence and to be truly objective when you have any kind of involvement in selling, manufacturing or promoting your own brand (which Paula obviously does). I don't mean this disqualifies anyone, but it's a tricky situation nevertheless. Paula's strenghts are her chatty style and engaging writing. I don't trust her to be of a particularly scientific mindset, although she does quote journals and other professionals in her work. Certainly, compared to most "beauty journalism" and "beauty writing", Paula's work is of some merit.
However, I don't really think there are any truly objective, thoroughly researched books on cosmetics and the industry out there (which is why I'm writing one, aha! However, I will have to dance on that awful "declaration of interest" fence too, as I am also involved in the making and promotion of products.
As with encyclopedias, history books and guide books on pretty much any topic - books on cosmetics are still subject to human error and to poor sources (I conduct research as part of my job and I have learned that it's wise to be paranoid about the accuracy of your sources). It depends on the author really, how much on "good faith" they take what a particular source might claim. I feel it is certainly a must to check any information you claim as factual at least twice; from sources independent of oneanother.
Traditional beauty journalism usually involves either printing a press release word-for-word without fact checking, or chancing a few words in a press release, then printing it without fact checking. Beauty editors are also duty bound to promote and push products and their department heads usually have a policy of "if you are going to say anything critical of a product, we won't print it. Positive, promotional blurb only, please!".
Most beauty books are written by beauty journalists, ex beauty journalists and other similar professionals, often working to a brief not too dissimilar from the one above.
That's where Paula's work stands out; she's gone out to at least have a go at separating fact from fiction. I do find it regrettable that there are errors in her work, as this has lead her to perpetuate some of the mis-information out there, which is a real shame.
A great deal of beauty writing is treated as opinion-based, of course, but because Paula pitches her work as a more fact and science-based look at what the ingredients are actually supposed to do, I do wish she had taken more care to remain objective and to educate herself a little more before publishing.