Altru loves for his followers to go and learn all they can to improve their spellcasting, especially if it leads to the creation of a new spell.  His follows tend to fascinate over new magic (even if its just new to them) and put forth great efforts to learn it.  His worshippers, therefore, include all types of spellcasters. His symbol, the open book, is meant to represent his love of knowledge, and willingness to share it.