All of the self-help books out there belong into the dubious category.
Whether it is about psychology, interpersonal skills, psychology, medicine or especially nutrition; and the biggest loser has to be economics.
Many psychology research findings are in a replication crisis, not the least because they are based on findings in weird people. Yet, the 'results' are gobbled up by popular books and remakers of ideology.
Medicine is as much an art form today as it was in the time of Imhotep. Sure, there is very hard science going around and many actors are performing in the best of faiths. But inherent limitations of smaller studies or big systematic reviews are seldom acknowledged by the popular media. Medical professionals should know that: Why Most Published Research Findings Are False. But on the bookshelf of a department store you will just find the opposite. I'd nominate any last one of those books that relates to "cure for cancer".
After proteins were discovered, people were told to eat meat, as that was supposedly good for them. Then vitamins were the cure-all for everyone and everything. But cholesterol was a new nemesis, scaring people away from eggs and meat. "Eat more carbs, eat no-carbs, saturated fat is good for you, cholesterol is harmless." We know just not enough about the complexities of nutrition and in developed countries people are well fed and mostly approaching an area of diminishing returns.
Looking at the evidence based reviews like at Cochrane we have to face the fact that indeed most dietary advice from the last 100 years was very dubious if it came from medical professionals. And advice that came from dietary zealots that screamed this time we've got it" fared way worse.
Given these broad categories to choose a wealth of nominees from, I'd suggest to consider the following:
Durk Pearson and Sandy Shaw: "Life Extension: A Practical Scientific Approach", 1982:
The book discussed free radicals and the idea that they cause aging, and how antioxidants were said to partially prevent the damage they do.3 The book suggests causes of aging and ways to slow them, with material on improving health and various aspects of the quality of life.
One notable feature of the book was several full-page pictures of its male and female authors, Durk Pearson and Sandy Shaw, striking bodybuilding poses and showing off some impressive muscles for "sedentary research scientists," which they claimed was due to the "growth hormone releasers" they took daily.
Medical health experts have dismissed Pearson and Shaw's life-extension formula as dangerous. The American Council on Science and Health reported that the risks of their formula included headaches, intestinal disorders and kidney damage.
Put anything "quantum physics" into a popular book, and see the disaster unfold:
Rhonda Byrne: "The Secret" 2006.
The book has been translated into 50 languages and has sold over 30 million copies. Due partly to an appearance on The Oprah Winfrey Show, the book and film had grossed $300 million in sales by 2009. Byrne has subsequently released Secret merchandise and several related books.
In 2009, Barbara Ehrenreich published Bright-Sided: How the Relentless Promotion of Positive Thinking Has Undermined America as a reaction to self-help books such as The Secret, claiming that they promote political complacency and a failure to engage with reality.
Byrne's scientific claims, in particular concerning quantum physics, have been rejected by a range of authors including Christopher Chabris and Daniel Simons at The New York Times and Harvard physicist Lisa Randall. Mary Carmichael and Ben Radford, writing for the Center for Inquiry, have also pointed out that The Secret has no scientific foundation, stating that Byrne's book represents: "a time-worn trick of mixing banal truisms with magical thinking and presenting it as some sort of hidden knowledge: basically, it’s the new New Thought."
A syncope opens up between professional psychologists and kitchen psychology with this 'masterpiece'.
Norman Vincent Peale: "The Power of Positive Thinking", 1952:
Peale's work came under criticism from various mental health experts, theologians, and academics. One general criticism against Peale's book was the lack of verified sources. The Power of Positive Thinking includes many personal anecdotes that the reader has no way of validating. The book includes stories about “a business executive”, “a man, an alcoholic”, “a famous trapeze artist”, “a friend of [Peale’s], a midwestern businessman”, and other unnamed individuals which cannot be verified from the information Peale presents with each anecdote.
[…] Murphy explains that such repeated hypnosis defeats an individual's self-motivation, sense of reality, and ability to think critically. He describes Peale's understanding of the mind as inaccurate and his description of the workings of the mind as deceptively simplistic and false. […] Ellis stated that eventually Peale's teachings “lead to failure and disillusionment, and not only boomerang back against people, but often prejudice them against effective therapy."
While contemporary theologians and mental health experts criticized Peale's teachings in The Power of Positive Thinking, the general public praised the self-help book. The Los Angeles Times estimates that “legions of followers testified that Peale’s message changed their lives for the better and represented the best combination of faith and pragmatism.”
This is evidenced by the popularity of Peale's book, which sold more than 5 million copies worldwide and was eventually translated into over 40 languages. In addition, Peale was close friends with American presidents Eisenhower and Nixon, both of which highly regarded his positive thinking teachings. Countless others accredited The Power of Positive Thinking for their success in overcoming obstacles including George Foster, of the Cincinnati Reds, Rev. Robert Schuller, founder and pastor of the Crystal Cathedral in Garden Grove, and Billy Graham, a prominent Southern Baptist Minister.
President Donald Trump has called Peale “his pastor” and “one of the greatest speakers” he had ever seen. Fred and Mary Trump, President Trump's parents, traveled to the Marble Collegiate Church in Manhattan with their children to hear Peale's sermons. Trump grew up hearing Peale's teachings from his parents, and Peale officiated his first wedding. Trump credits his survival in 1990 after bankruptcy to Peale's positive thinking teachings.
And some will argue that the following doesn't even really count, but it claims to be based on science, in its title, and has had a huge influence, with a publication run of more than 83 million copies sold:
L Ron Hubbard: "Dianetics: The Modern Science of Mental Health", 1950:
Although it received an initial positive public response,8 Dianetics was strongly criticized by scientists and medical professionals for its scientific deficiencies. The American Psychological Association passed a resolution in 1950 stating of Dianetics "the fact that these claims are not supported by empirical evidence of the sort required for the establishment of scientific generalizations."