What is the ruling on someone who tells another person about a foreign book and then absolves himself by mentioning the existence of transgressions in it? And what should he do if his action is impermissible?

If a book contains both benefits and objectionable matters, and you guide to it while warning against the objectionable parts, then there is no harm. However, if the benefits can be found in other books that are free from objectionable content, then guiding to those books is preferable. And if you strongly suspect that the person you are guiding will be harmed by the objectionable matters and will not avoid them, then it is advisable to refrain from guiding them to that book, because averting harm takes precedence over achieving a benefit.