First comes to my mind this fun article I recently read about Nietzsche and marriage:
"10 tips for a great marriage according to Nietzsche", Skye Nettleton, Indo-Pacific Journal of Phenomenology, Volume 9, Edition 2, 2009.
In Human, All too Human, Nietzsche suggests that it
would be much better (for men, presumably) to do away with the custom
of one wife for life and instead “one might very well consider whether
nature and reason do not dictate that a man ought to have two
marriages” (p. 156). The first marriage is the most important and
necessary for a man’s education; it should be when the man is
twenty-two years old to a woman who is “intellectually and morally his
superior and who can lead him through the perils of the twenties”
(Nietzsche 1878-80/1996, p. 156). A second marriage, while useful, is
not necessary; it should be during a man’s thirties and to a younger
disciple “whose education he would himself take in hand”. Later in
life, man should preferably be without a wife because marriage “is
often harmful and promotes the spiritual retrogression of the man”
(Nietzsche 1878- 80/1996, p. 156). In a later work, Nietzsche cites a
raft of great philosophers who have not been married as evidence for
this incompatibility between marriage and personal fulfilment:
“Heraclitus, Plato, Descartes, Spinoza, Leibniz, Kant, Schopenhauer”,
with only Socrates as the ironic exception (1887/1989, p. 107).
Referring to:
- Nietzsche, F. (1989). On the genealogy of morals (W. Kaufmann & R. Hollingdale, Trans.). In W. Kaufmann (Ed.),
On the genealogy of morals & Ecce homo. New York: Vintage Books. (Original work published 1887)
- Nietzsche, F. (1996). Human, all too human (R. J. Hollingdale, Trans.). Cambridge, UK: Cambridge University
Press. (Original work published 1878; additions in 1879, 1880).
Additionally I remember having seen a book called "Solitude: a philosophical encounter", Philip Koch, Open Court Publishing Company, 1994. It was very long reflection about solitude, and its many different aspects. I did not read it, so I am not sure about the contents, but it certainly is talking about what you are interested on.
A quote from the chapter 7, "the virtues of solitude":
I believe that most people find—have always found—solitude
particularly propitious for achieving the values I will be
enumerating; but some people simply don’t find those values best in
solitude. One might try to defend the sharp generality of the virtues
of solitude by arguing that unappreciative people are alienated,
psychologically blocked, spiritually emaciated, etc., and, although I
tend to believe this, I am doubtful that such arguments would be
decisive for every case. Better, perhaps, to draw an analogy to the
virtues/values of works of art: most people find Rembrandt’s portraits
quietly moving but some do not, and most people find Greek temples
inspiring though some do not. The numbers of the unappreciative can be
greatly reduced by patient education, but a few unregenerates will
remain. The fact that there are a few such people, however, does not
destroy the virtuosity of those artistic works, their lustrous
richness and timeless nobility; just so, the fact that a few people do
not observe nature best alone does not prove that attunement to nature
is not a special virtue of solitude. That it is such a virtue, the
following pages of celebration and reflection undertake to establish.
Anyways, I can tell you also that solitude is also the thing that is very feared by all those that try to avoid, or feel overwhelmed by, discussing solipsism, which is the idea that maybe I am the only thing that exists. You can find thousands (not joking) of academic texts written usually by persons that give all kinds of motives, usually not logically valid, to avoid it, and to assume it is false. It is quite evident that they fear it be true, as there is only a very tiny quantity of philosophers that actually dared to treat this issue with the rigour it deserves (or that it needs to actually research it), instead of just saying "that's for mad men", as even Schopenhauer did, even though he did not seem to be afraid of facing any other philosophical problem.