Besides the many Inscriptions, bas-reliefs and frescoes found on temple walls and tombs, we have written sources, too, providing us with information about Ancient Egypt's temples and priests.
A recently discovered, but not yet completely restaurated and translated source is the so-called "Book of the Temple". There are fragments known in many museums of the world (Aberdeen, Ann Arbour, Berlin, Florenz, Heidelberg, Kopenhagen, London, Manchester, New Haven, Oslo, Oxford, Paris, St. Louis and Wien), written in Hieratic, Demotic and Greek. The content of the text is much older than the fragments themselves, and scientists believe it might have been in use for hundreds of years. The "Book of the Temple" contains detailed information about how a temple should be built and administrated, but also regulations for the priests.
In using his material, we have to keep in mind that Herodot wrote in the 5th century B.C., already in the late period of Egyptian culture and beliefs, and he got his information highly probably only second hand. Herodot himself did not even speak Egyptian. He took anecdotes and stories in his account, not unlike modern travel writers do.
He was a Greek historian and philosopher, who partly lived in Egypt and wrote on demand of Pharaoh Ptolemy I. He idealises the Egyptian culture and tells history in a way, as if all science and culture had its origins in Egypt.
Diodorus was Greek, however lived some time in Egypt. In his historical work, he claims not to have used the 'fabulations of Herodot and other writers', but to have done research in the documents provided by the Egyptian priests themselves. Nonetheless, Diodorus used some not very accurate sources. Amongst others, Hekataios of Abdera's writings belong to his sources.
Plutarch was a Greek writer and historian. In his work "About Isis and Osiris", he informs about that cult - the most important Ancient Egyptian cult in hellenist times - and its priests.
Porphyrios, probably originate from Syria, studied in Athens and later lived and died in Rome. He was a philosopher of great renown, and sharply criticised the Bible and Christianity. In one of his works, he praises an ascetic, vegetarian lifestyle of Egyptian priests. In a (fictional?) letter to an Egyptian priest, he asks questions about Egyptian religious practics and cult and demonstrates his critical viewpoint as well.