Is the book: A Course of Pure Mathematics (G H Hardy) still relevant today? A very intelligent family member of mine has recently decided to send me a large number of books and other resources on mathematics, include Hardy's book. Since this was published in 1921, is it still a useful textbook from which to learn pure mathematics?
Congratulations to such generous and wise members in your family. My answer is: Yes, of course. This text is great to read and will  help  to  gain  a deep and  proper understanding of the material.

Hint:  In order to better evaluate this treasure, the following might help to come to a decision by yourself:

Review 1 by Paramanand Singh, one of the members of this platform.

Review 2 from MAA.

Proposal: Select two, three sections from Hardy's book which you might find interesting, read them and compare it with corresponding sections from books you are familiar with.
It depends on your taste and interests. Many old books are fantastic, the age is not a problem. I personally dislike Hardy’s book and Hardy himself because I do not believe in “pure Mathematics”.  Hardy’s disregard for applications seems to me arrogant and childish. You can learn Math from many other books that include nice motivations from Physics, Engineering etc. The older texts by Hermite, Goursat etc. Are more interesting in my opinion.