St Theophan the Recluse, after advising a spiritual daughter about the when, why, and how of reading spiritual literature, adds this:
The question still remains unresolved as to whether one may read anything besides spiritual things. I would tell you with reservation, in a low voice: You may if you like, but just a little and not indiscriminately. Take this as a sign: When you are in a good mood spiritually and begin reading a book containing human wisdom, if the good mood begins to desert you, get rid of the book. This is a general rule for you.Excerpted from "On the Reading of Spiritual and Secular Books" (letter number 70) in The Spiritual Life and How to Be Attuned to It, pp. 285-287.
Even books containing human wisdom may nourish the spirit. These are the books that indicate to us the vestiges of wisdom, benevolence, truth and solicitous Divine industry in nature and history. Read books such as these. God reveals Himself in nature and history in the same way as in His Word. Nature and history are Divine books for those who know how to read them.
You will say, "It is easy to tell me to read such books, but where do you get them?" This I cannot tell you. There are presently more books on scientific subjects coming out. Most of them, however, have a very bad direction; namely, they attempt to explain the origin of the world without God, and all moral-religious and other manifestations of the spiritual in our lives without the spirit and soul. Do not get those books. There are books on scientific subjects without such false wisdom. These you may read. It is good to understand the structure of plants and animals, especially man, and the laws of life as they are manifested in them. Great is the Divine wisdom in all of this! Unknowable! Ask the fellow who enjoys discussing religious subjects which books are of this sort.
What about stories and novels? There are good ones among these. To find out whether they are good, however, you must read them, and after you are finished, you will have acquired such tales and images that – God have mercy! You will soil your clean little mind. Afterward, go get cleaned up. Why would you want to bring such labor upon yourself? Therefore, I think it best not to read them. When a benevolently-minded person who has read some story recommends it, you may read it.
There are good descriptions of geography. You may also read these. However, do a little of everything, and then only for variety. Keep up your spiritual work, and do not turn your attention away from it.
May the Lord bless you.