I will divide my answer into three parts. Connections between:
1) Maids and minorities
Ehrenreich admits that most of the women she worked with in maid-service were white, and that fact seems to have been substantiated by the Bureau of Labor Statistics' numbers. However, the association of house-keeping with minority status has been well-documented within white-employer class circles. She shares anecdotes about assumptions regarding racial stereotyping in maid service. However, she also goes on to state that the racial composition of workers in maid service tended to depend on the availability of economic opportunity.
For example, Irish and German immigrants served the upper classes during the 19th century and up until the 1940s, maids were predominantly African-American in the United States. These groups of people left employment in maid service and housekeeping jobs when economic opportunity opened up for them in better paying careers. So, one can postulate (assume to be true or claim) that maid-service workers tended to be of a certain racial or ethnic make-up until better economic opportunities opened up more employment options for them.
2) Maids and poverty
Ehrenreich asserts that housekeeping employment perpetuated poverty because of low pay and even worse benefits. She mentions that an independent cleaner makes about $15/hr while an employee for The Maids gets paid $6.65/hr. The Maids company earns $25/hr. Even accounting for the independent employee having to apportion out a certain amount for taxes (The Maids company also has to do the same), this sum is paltry, considering the back-breaking work required of maids who work for the company.
Ehrenreich's co-workers struggle to survive; many are rooming with extended family and house-mates. Half-smoked cigarettes are carefully saved for later. On the way to jobs, the maids argue about how they can come up with enough money for tolls. One co-worker frantically tries to locate sources for free dental care in order to deal with a painfully impacted wisdom tooth.
3) Maids and invisibility
Ehrenreich asserts that maids are often marginalized in society; their careers are considered a type of low-skill employment which requires little or no thought. Ironically, Ehrenreich finds that even restaurant workers/waitresses making the same low wages tended to provide unsatisfactory customer service to an employee in a maid-service uniform.
Ehrenreich also cites evidence that in the larger macroscopic sense, household workers represent a largely uncounted class within the American economic system: Zoe Baird, an American lawyer, lost her chance to become the United States Attorney General in 1993 due to Nannygate. Many of those hiring illegal immigrants as maids tended to hide this fact from the IRS, preferring to pay under the table in order to avoid higher costs (taxes, etc.) and to avoid prosecution. This leads to the marginalization ("invisibility") of maids, both socially and economically.