The Sefer Matamim is written by Rav Yitzchak Lipetz of Shedlitz. His work provides fascinating reasons for various customs throughout the Jewish calendar, and is occasionally quoted in various sources. However, this site seems to note there was a well-known maskil named "Yitzchak Lipetz" from Siedlce. Was the author of the Sefer Matamim among the maskilim, and if so is it better not to publicly teach the sefer?
This blog post brings information about him. In the post it says:
"...he was born in Shedlets (Siedlce), Poland. He attended religious elementary school and yeshivas, acquired the reputation of a prodigy, and prepared to become a rabbi. After the traditional period of support from his father-in-law, he became a bookseller and worked at it until he was quite old. He was the founder of the first Jewish lending library in his city, and although a religious man himself, he nevertheless influenced youngsters in the spirit of the Jewish Enlightenment.
Edit: Having searched through more of your link, I see that these two were the same person, as it says:
"A library existed in Siedlce from the end of the nineteenth century. The owner of this library was Yitzchak Lipietz, a book dealer, who would sit between minchah and ma'ariv in the assembly room and study with the everyday Jews the portion of the week. Lipietz was a maskil, a writer of popular books and commentaries, someone who knew current Haskalah writings, medieval Jewish poetry, and Jewish philosophy. He had collected about a hundred books and lent them to readers for a weekly payment."
As for your other question, on the practical side it might be a CYLOR, but from a more theoretical perspective - I would hazard to say that it depends if you actually find anything problematic within the book.