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)?

**Addendum.** I am asking this question not because it is something that I am thinking about doing. Instead, I am asking it because this practice is probably not something that people would openly admit to having done, yet I have the sense that it is not uncommon. (Indeed I was motivated to post this question because I just happened to come across a clear instance of such practice.) I simply wanted to know: To what extent is it frowned upon or regarded as unethical? Presumably on the scale of unethical behavior, it is considered less unethical than sexually harassing your students. But how much less so?