Years ago Pentecostal preacbers would mainly read the Bible, pray and fast with little or no reading of supplementary books.
Due to lack of good books from Pentecostal writers and more difficult circumstances of living they were much more dependent on God.
Since their study and prayer came directly from the Bible it was reflected in their preaching, their experience and the results of their preaching.
It reflected their Pentecostal beliefs.
Now there are many more books available and as a result much of the time the emphasis of the Bible is shared with books, sermons, etc. with less of an emphasis on prayer and fasting.
Books are good, can be a great blessing and the ministry was ordained by God to perfect, train and equip the saints for the work of the ministry (Ephesians 4:11-15).
The problem though now is there is still a great shortage of books by Pentecostal writers and most of the books and authors read by Pentecostal preacbers are non Pentecostal written from a non Pentecostal view.
Over time this non Pentecostal view is reflected in the preaching, experience and results of the preaching.