Case:


Ruling: [Without getting into details,] professionals confirmed that def copied straight from pl’s edition and then added in changes to "cover his tracks." Pl demonstrated how his work is qualitatively different from previous editions, whereas def’s edition has only cosmetic changes.
Are there copyright rights in regard to improved editions of old books? Let us consider the source of such rights in general. The Chatam Sofer writes that the prohibition on infringing on a sefer’s author’s rights is a special institution so that those who are involved in a mitzva are not harmed. He also points out that a Jew who tries to remove his counterpart from a job he began is called a wicked person, where the following Rama (Choshen Mishpat 386:3) provides classical precedent. Reuven lent money (to a non-Jew) with interest and Shimon convinced the borrower to return the money and borrow from Shimon. While the Rama says that Shimon does not have to pay, he says that he is called wicked. This is all the more so in our case where pl is an avrech whose main livelihood is from sales of his work, and they have been significantly reduced since def’s edition came out.
The Chatam Sofer (Shut CM 79) addresses a similar case of someone who published a previously written work and explains why other people are forbidden from putting out similar editions. While it is not naturally forbidden, it is logical to forbid it because he relies on the expectation that others will not come and cut into his sales. The Pitchei Choshen (IV, 9:2) wonders whether that assumption applies in our time when seforim are republished on a regular basis. However, the limited scope of the prohibition applies only to a simple reprinting, not to one which qualitatively improves the old edition and then someone "steals" the additions.
The Shoel U’meishiv (I, I, 44) says that an original author has exclusive rights, based on hasagat g’vul (the rules of encroachment), to publish for as long as he wants, even without rabbinical bans that are often added. The reason time limits on the ban used to be given is because after the author sells his stock he is happy that others spread his work. In the case of reprinting old works, assuming inheritors cannot be identified, one who prepares it for publishing can forbid others from essentially photocopying that which he did.
Therefore, def cannot print or sell any more volumes. Pl is willing to buy outstanding copies from def at price value, and def must reimburse pl a reasonable amount for past damage.