これはアメリカ人にも読める日本語の本です。
I received this sentence as an example of に being used with potential form in a passive sense, but even though the person said they were Japanese (online) I have never seen anything like this ever.
What is this grammar point? Can I have any other examples? If it is incorrect, how would you say "Y can be done by X" where it is both potential and passive?
Usually, when people say "passive" in the context of Japanese, they're describing verb forms that have the suffix 〜(ら)れる attached. Take a look at the following examples, taken from the section on passives in Shibatani's The Languages of Japan, p.318:
友達【ともだち】が太郎【たろう】を殴る【なぐる】。 (non-passive version)
"The friend hits Taro."太郎は友達に殴られた。 (passive version)
"Taro was hit by a friend."
This form can express a number of things, including passive, potential, and less commonly honorific and spontaneous meanings. For details on this form, please see your textbook; in this answer, I'm going to concentrate on the form you asked about.
Since your sentence doesn't contain 〜(ら)れる, I would say it's not actually passive. You could argue otherwise, and you could translate it with the passive voice in English if you like, but I think it's active in Japanese.
In your sentence, に is expressing who is capable of reading the book. It changes the statement from a general one to a statement about Americans. (But since there's a も, it means Americans as well [as Japanese], or even Americans.)
Take a look at the following examples from Shibatani's 1999 paper on dative constructions, pp.63-64:
a. 日本語が話せる。
"Japanese can be spoken."b. ハワイで(は)日本語が話せる。
"In Hawai'i Japanese can be spoken."c. ケンに(は)日本語が話せる。
"Ken can speak Japanese / Ken can be spoken Japanese to;
(lit) With respect to Ken, it is true that Japanese can be spoken"
Here's what Shibatani write about these examples:
Consider a Japanese potential expression like [a]. It is not true that Japanese can be spoken anywhere or by anyone. This statement thus needs to be confined to a particular domain. This can be done either by providing a location in which Japanese can be spoken, as in [b], or a person who can realize the potential state, as in [c]. (emphasis added)
The statement you're asking about is similar to [c], except that it's been turned into a relative clause. Here's a non-relative version, with the head noun phrase 日本語の本 inserted with a が-role:
アメリカ人にも（日本語の本が）読める
"A Japanese book is readable by { even Americans / Americans, too }"
This noun phrase is pulled out, and it's turned into a relative clause:
（アメリカ人にも読める）日本語の本
"A Japanese book that { even Americans can read / Americans can read, too }"
This makes a single noun phrase which is placed inside the larger sentence これは〜です, which of course means "This is 〜 (polite)":
これは（（アメリカ人にも読める）日本語の本）です。
"This is a Japanese book that { even Americans can read / Americans can read, too } (polite)"
And with that, we've put your sentence back together.
The potential form is the potential form, and has no special meaning or influence in this pattern. As @snailboat mentioned, there is no passive form anywhere in this example. I think the core of your confusion is the presence of the に. For that, I'll refer you to this other topic: が and に interchangeability and difference in meaning.
This is a case where に is replacing が and it seems somewhat unnatural. However, as I explain in my answer in the above topic, this に translates to something like "unto ～" or "by ～". So the following clauses are essentially interchangeable:
- アメリカ人が読める日本語の本 → A Japanese book that Americans are able to read
- アメリカ人に読める日本語の本 → A Japanese book that is readable by/unto Americans
Now, adding the も is just adding the emphatic "even ～". In this case, the が version of the clause would replace the が with just も, but the に version would add も to get にも.
- アメリカ人も読める日本語の本 → A Japanese book that even Americans can read
- アメリカ人にも読める日本語の本 → A Japanese book that is readable even by/unto Americans
(Note that with using "even" in the が version, I believe you could technically change it to be the double-/compound- particle もが ― アメリカ人もが読める日本語の本 ― but saying it that way seems very literary and/or archaic. Someone please correct me if I'm wrong on that.)
So I think the root issue here is the が/に swap. Hopefully this clears it up for you.
