At Discussion Group, I'd offered my process of how I deal with friends involved with false teachers, false doctrine, or false religion. I'd said that first, it depends on the relationship you have with them. If you don't know the person or are only bare acquaintances, it won't do to walk up to them and just say something brusque or out of the blue that in effect, amounts to saying "You're doing it wrong."
The Bible encourages and commands discipling relationships with one another. This is so we can keep each other accountable. We can carry each other's burdens. Ultimately, close involvement with each other means can edify and grow one another and one way we do this is by helping sisters course-correct.
Therefore encourage one another and build one another up, just as you are doing. (1 Thessalonians 5:11).
We can't build up a sister if we let them wallow in false doctrine. (Jude 1:23). Alternately, we won't build them up if we are tactless and brusque. (2 Timothy 2:25, Galatians 6:1).
Assuming you are close enough with the sister to have established trust and are known to each other in a friendly way, then what I do is begin by asking questions. Perhaps the person is reading the book for research purposes. Maybe someone less discerning gave it to them and they haven't thrown it out yet. Maybe they are getting ready to give it to someone else. Maybe a lot of things. Just ask. "Are you reading that book? What do you think of it?" "Let me know when you're done, I'd love to get your take on it..." Etc.
Finally, I always have something else to offer the person in the books' stead. It doesn't help the person as much to just say that their book is a dangerous book, without having something in which to substitute. If they'd lacked discernment enough in the first place to get or read that book, then offering them material written by a credible author steers them into a better direction.
I was pleased when I'd come across this short discussion from 2016 where Noël Piper, Kathleen Nielson, and Gloria Furman discuss this very question. I was even more pleased when they shared that they do the same: ask, be gentle, discuss. Phew, at least I'm not off the deep end with this.
The women also discuss two other questions. If you can't play back videos, here is a link to just listen.
One final thing. Their title mentions 'a dangerous book." Undoctrinal books ARE dangerous. Books like The Shack, Love Wins, The Circle Maker, any and all non-doctrinal, unorthodox books present a danger to the Christian. Adam and Eve only had to obey one command, and within a shockingly short time, satan easily managed to twist that command into a suggestion. Paul said to Titus that false doctrine upsets whole families (Titus 1:11). Paul warned Timothy that false doctrine undermines the faith. (2 Timothy 2:18). Make no mistake, (because satan isn't making the same mistake), the false doctrine contained in books, movies, pamphlets, and studies is a very present danger to the Christian mind and heart.
Here is the blurb for the short video:
Your friend is gushing about that book she’s been reading. It’s on the Christian Living bestseller list, but for whatever reason you suspect the book is more influenced by the spirit of the age than by a biblical worldview. ... Nielson cautions against the overcorrection of reading only the Bible, since reading widely can actually enhance Bible reading, and Piper warns against becoming the kind of reader who only reads books from your own "tribe."
Help! My Friend Is Reading a Dangerous Book
Noël Piper, Kathleen Nielson, and Gloria Furman Discuss