Large book certificate. If a person is really seriously interested in psychology, then he probably already has a list of several dozen books that he would very much like to read, it is better not to interfere with the selection process, but simply give the opportunity to make it.
The Collected Works of CG Jung in 20 volumes.
As the most commonplace, but the correct option is a book. Considering that nowadays a lot of good literature is being produced that cannot be found on the net, and printed books are not cheap, such a gift will be very useful. The most important thing is not to be mistaken with the topic, because hobbies in the field of psychology can also be different. There are some curious books about the work of the brain that will certainly be of interest to anyone interested in psychology. For example, "I am an illusion, or the games the brain plays with us."