The word "manifesto" is a bit provocative. This book is a little more thoughtful than that.
The basic premise, which I agree with wholeheartedly- is that voluntary solitude is a natural, nonpathologic orientation held by a minority of the population. Unfortunately, it is very misunderstood, and solitude (voluntary, involuntary, or somewhere in between) is also exhibited by people with assorted personality disorders: agoraphobics, outcasts (for a wide spectrum of reasons), extreme shyness, and criminal types with something to hide. The tragedy is that voluntary, nonpathologic "loners" are often lumped in with the criminals, being perceived as having something essentially damaged about them, which requires medical cure, social intervention, or even legal correction.
Most of the book explores the contributions of nonpathologic "loners", some of whom have been celebrated as extraordinary thinkers and creative artists (Emily Dickinson, Albert Einstein, Marie Curie, Pablo Picasso, etc). It's not just a list of celebrities here, but lesser-known explorers (Admiral Byrd lived for 4 months in solitude in Antarctica, establishing the first weather station there, when the rest of his party had deserted him), people who have undertaken solo around-the-world sailing journeys, etc.
The point is that many of these people have undertaken worthwhile projects, and have contributed significantly to society, yet whenever there is a mass shooting or some other tragic event, the perpetrator is almost always characterized as a "loner". With just a little bit of research, though, Ms. Rufus demonstrates that most of these so-called "loners" fall into one of two groups which are decidedly not the same as the creative, elective solitaries described above. The "lone nuts" are often either (1) social people who have been outcast, and who are not comfortable with a solitary lifestyle... people who have been rejected by their peers, and who feel pain and resentment over this (something an elective solitary would never feel); or (2) social people who were actually very well integrated into their communities, and who enjoyed a circle of friends, but who are nevertheless characterized as "loners" after their crimes, either because their social circles are not well-known to investigators, and/or because the friends and family abandoned upon discovery of their criminality.
Why would social people be mischaracterized as "loners" so frequently and predictably after their crimes? Rufus explores this, and makes a convincing case that the shunning and stigmatization of loner behavior is a societal defense mechanism. So many of the vital functions of society require fellowship, community, and congregation. Tolerance or encouragement of loner behavior would undermine this. There is an entire chapter on religion, and how all of the major religions telegraph lessons about how "proper" religious practice requires group worship. Think of all religious festivals, the way holidays (as in holy-days) have been celebrated through the ages... how all important religious milestones are attended and recognized with formal gatherings (baptisms, bas mitzvas, marriages, funerals, etc) A passage in the Bible (Matthew 18:20) states "...For where two or three are gathered together unto My name, there am I in their midst." Holy cow! You'd think somebody shipwrecked alone on an island, or thrown into solitary confinement would be cut off from the Divine Creator! Does anybody really believe in such a god?
Another function served by the mainstream narration that every gunman and serial killer is a "loner" is the soothing misconception that "I don't know anybody like that". The disturbing truth is that most serial killers (and several are examined here) are all the more terrifying exactly because they hide so well in plain sight, and that they literally were at some point somebody's "friendly neighbor whom [you'd] never have suspected". John Wayne Gacy was a well-liked member of the Shriners, who regularly visited sick kids in the hospital, and who was once voted "Citizen of the Year" by his local Jaycee's. He was the life of the party, and socialized frequently. Who knew he had 34 bodies stashed in his crawlway? It would be so much more comforting to think Gacy's crimes could only be perpetrated by that only the weird, withdrawn old man who seems to sleep all day and whose lights are on all through the night; or maybe the crazy irritable widow down the street who lives with 100 cats. Unfortunately those stereotypes are just that: misleading stories the public tells itself so it can sleep better. ...And it isn't just Gacy who is shown to have been social; examination of Timothy McVeigh's life shows he had a reasonably broad circle of friends and liked to socialize; likewise for the Columbine shooters, the San Diego school shooter of May 2001; and the duo convicted of random shootings around Washington DC in October 2002. All of these were incorrectly characterized as "loners" by the media.
Is all of this a bunch of hair-splitting, or is this significant? I'd say significant. The final chapters are tragic, if you can relate to the loner's orientation. There seems to be a trend in current-day childrearing (at least in the United States) which is forcing children into more and more highly-structured social activities ("play dates", organized sports, after-school clubs), thus diminishing the time an more solitary-oriented child can actually spend alone. Teachers are trained to recognize the child who can productively and creatively enjoy his/her alone time as being pathologic... in need of correction. A child who doesn't gravitate to group activities is identified as depressed, ADHD, or some similar label is applied. The parents are brought in for counseling, and taught ways to "persuade" the child to get "back on track". It's heartbreaking. If you've ever been in subject to such treatment, you can identify. The book actually ends on a bleak note, wondering if the next Marie Curie or Albert Einstein will have their creativity pounded out of them with endless miserable Cub Scout meetings, Little League games, "play dates", etc.