Secondary Sources

In a post on the Seforim blog, R. Eliezer Brodt points out instances in which an author evidently utilizes the sources found in an earlier work without giving attribution (link). The question I raised in the comments is whether that practice is contrary to halakhah. I later remembered that R. Aaron Levine discusses almost this exact question in his Moral Issues of the Marketplace in Jewish Law (pp. 31-35). R. Levine's case is of a rabbi who bases a lecture on sources quoted in the book Beris Yehudah but fails to tell people that he received assistance from Beris Yehudah. R. Levine concludes that such a practice might be considered geneivas da'as (deception), depending on the common assumptions of readers. It definitely contravenes the obligation of gratitude to teachers, as defined in Nazir 56b to require naming the first and last in a chain of teachers. See the excerpt in this post (link).

Click here to read moreR. Simcha Rabinowitz (Piskei Teshuvos vol. 2 156:27) quotes R. Shlomo Kluger's Chokmas Shlomo glosses to Shulchan Arukh (Orach Chaim 156) as reaching same conclusion as R. Levine. But in footnote 244, R. Rabinowitz quotes R. Yaakov Yisrael Kanievski as saying that you are not obligated to say where you saw a source quoted but it is a proper practice to do so (as quoted in Toledos Ya'akov, ch. 15).

On plagiarism in general, the She'arim Metzuyanim Ba-Halakhah (on Yevamos 91a sv. a"l Rav Nachman) cites Magen Avraham 156:2; Noda Bi-Yehudah (2:OC:20); Shakh (Yoreh De'ah 242:43); Responsa Machaneh Chaim (Choshen Mishpat no. 49); and Responsa Maharam Shick (Yoreh De'ah no. 156).

See also this post: link