Some of the works of the author Haroon Yahya are educational and beneficial, but he has also written on the subject of evolution.

Answer

If even one of his works is haraam/kufr such as the kufr of evolution, then it is Waajib to refrain from all his books. It is not permissible to read/study the works of deviates who propagate kufr. Even the 'educational' articles of a deviate exercises a spiritually detrimental effect on the reader. Kufr and Fisq are extremely contagious.