The answer to this question is somewhat complex and depends on the country whose copyright laws you are talking about. Most copyright-abiding countries have adopted a copyright length of Life + 70 years, but several (most notably Canada) use Life + 50 years. However, there are a lot of exceptions in many of these countries regarding works created in the mid-20th century. The US, incredibly, has a copyright of 95 years for works published between 1924-1977, but a term of Life + 70 years for works published after that.

Since the beginning of this year, the original German edition of the book that was published in 1924 is in the public domain in the US (and most likely only the US). Unfortunately this applies only to this edition and not to any translated versions.

I looked around the front matter of the book and did not see any mention of a translator; presumably this means that Courant translated this himself. This means as soon as 50 years has elapsed from Courant's death (in 1972), all editions will be in the public domain in Canada and a few other countries (in 2023). In most other countries, this book will be in the public domain 20 years later than that (in 2043). The US is once again exceptional, where each edition will enter the public domain separately, 95 years after their publication. The English version of Volume 1 of this text in the Amazon preview appears to have been published in 1953, which would mean it would be in the public domain in 2049, well after it would be available in virtually every other country in the world.

Of course, all of this discussion above assumes that copyrights don't get extended again before these works enter the public domain. The US Supreme Court has ruled that arbitrarily extending copyrights before they expire is sufficient to satisfy the "limited Times" mentioned in the Constitution; what matters is apparently that the copyrights are set to expire at some point, not that they ever actually do.