All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallAllaahu ‘alayhi wa sallam, is His Slave and Messenger.
There is no harm in an author who wrote a book on beneficial topic to use a pseudonym as long as this does not incur any religious violation. The basic principle is that matters are lawful unless proven otherwise by Sharee'ah evidence.
As long as there is no (real) need for using a pseudonym, it is better for the author to publish the book under his real name (instead of using a pseudonym); perhaps some of those who would benefit from the book and seekers of knowledge would need to consult him or inquire about any issue about which they may get confused with regards to the information in his book. And there are other possible benefits gained by publishing the book under the author's real name.
Allaah Knows best.