Note: Though this was written to priests it is still very applicable to even the average lay faithful. We are all called to be saints after all.
Excerpt from the Apostolic Exhortation Haerent animo by Pope Saint Pius X
It is of great importance that the priest should combine his daily divine meditation with the constant reading of pious books, especially the inspired books. That was the command that Paul gave to Timothy: Attend unto reading.[1 Tim. 4:13.] The same lesson was taught by St. Jerome when instructing Nepotianus on the priestly life: “Never let the sacred book leave your hands”; and he gave the following reason for his advice: “Learn that which you are to teach; holding to that faithful word which conforms to doctrine, that you may be able to exhort with sound doctrine, and refute the opponents.” What great advantages are gained by priests who are faithful to this practice! With what unction they preach Christ! Far from flattering and soothing the hearts and minds of their audience, they stimulate them to better things, and arouse in them the desire of heavenly things.
The command of St. Jerome: “Let the sacred books be always in your hands,”[Ep. LVIII ad Paulinum, n. 6.] is important for another reason also, a reason which concerns your own personal welfare.
Everyone knows the great influence that is exerted by the voice of a friend who gives candid advice, assists by his counsel, corrects, encourages and leads one away from error. Blessed is the man who has found a true friend;[Ecclus. 25:12.] he that has found him has found a treasure.[Ecclus. 6:14.] We should, then, count pious books among our true friends. They solemnly remind us of our duties and of the prescriptions of legitimate discipline; they arouse the heavenly voices that were stifled in our souls; they rid our resolutions of listlessness; they disturb our deceitful complacency; they show the true nature of less worthy affections to which we have sought to close our eyes; they bring to light the many dangers which beset the path of the imprudent. They render all these services with such kindly discretion that they prove themselves to be not only our friends, but the very best of friends. They are always at hand, constantly beside us to assist us in the needs of our souls; their voice is never harsh, their advice is never self-seeking, their words are never timid or deceitful.
There are many striking examples of the salutary effects of the reading of pious books.
Outstanding is the case of Augustine whose great services to the Church had their origin in such reading: “Take, read; take, read; I took (the epistles of Paul the Apostle), I opened, I read in silence; it was as though the darkness of all my doubting was driven away by the light of peace which had entered my soul.”[ Confessions, L. VIII, C. 12.]
In our own day, alas! it is the contrary that happens all too frequently. Members of the clergy allow their minds to be overcome gradually by the darkness of doubt and turn aside to worldly pursuits; the chief reason for this is that they prefer to read a variety of other works and newspapers, which are full of cunningly propounded errors and corruption, rather than the divine books and other pious literature.
Be on your guard, beloved sons; do not trust in your experience and mature years, do not be deluded by the vain hope that you can thus better serve the general good. Do not transgress the limits which are determined by the laws of the Church, nor go beyond what is suggested by prudence and charity towards oneself. Anyone who admits this poison into his soul will rarely escape the disastrous consequences of the evil thus introduced.