Suppose I see that book X is cited several times with reference to some particular fact Y.

For whatever reason, I am not able to get my hands on book X (or perhaps it just takes me more effort than I am willing to expend).

Nonetheless I wish to make a mention of fact Y. And as far as I know, book X is the only reference for fact Y.

Would it be ethical for me to cite book X in my paper, even though I have never even caught a glimpse of that book and have no idea what it actually says (other than what I've seen reported in other works)?

I have the sense that this practice is not uncommon and it is not something that people would openly admit to having done, but I wanted to know to what extent it is frowned upon or regarded as unethical.