William Pannapacker, a professor of English at Hope College, has written at length about the state of doctoral education in the humanities. Read what he has to say here, here, and here. Then read what economist Dave Colander has to say about the job market for English PhDs, which is also relevant to disciplines across the humanities and social sciences: the article is here and there's a nice writeup of the article's main points here.
Because a PhD is ultimately a path to employment, you should be aware of job placement rates in your prospective field: the MLA has numbers on this for humanities disciplines PhDs here, NSF has numbers for different disciplines here, and the Atlantic pulled together numbers from various sources here. Related to this, before applying for a PhD you should also see what kinds of jobs are available and what their listed qualifications look like: for linguists like me, that means looking at the job postings on the Linguist List, the LSA's Career Center, and the Academic Jobs Wiki, but most professional organizations offer similar resources (e.g., the MLA, NCA, ASA, AAA).
You should also know that there are significantly more people getting PhDs than there are academic jobs: the numbers vary by field, but you can see an example of this trend in science and engineering here. If you are interested in a job teaching and researching at a university, you should read historian Erin Bartram's very frank discussion of her decision to leave academia after years of (unsuccessfully) trying to get a permanent university position, available on her website here. You will also want to learn more about the so-called "adjunct crisis" in higher education: read what the AAUP has to say here as a starting point. Finally, if you are serious about applying for a PhD, you should also know about mental health issues in academia: two reports on mental health issues for graduate students can be found here and here.