Books and most importantly the Scriptures.
2 Timothy 4:13:
"When you come, bring the cloak that I left with Carpus at Troas, also the books, and above all the parchments."
Here's an excerpt from Spurgeon's sermon on this text:
We do not know what the books were about, and we can only form some guess as to what the parchments were. Paul had a few books which were left, perhaps wrapped up in the cloak, and Timothy was to be careful to bring them. Even an apostle must read. . . . A man who comes up into the pulpit, professes to take his text on the spot, and talks any quantity of nonsense, is the idol of many. If he will speak without premeditation, or pretend to do so, and never produce what they call a dish of dead men's brains—oh! that is the preacher. How rebuked are they by the apostle!
He is inspired, and yet he wants books!He has been preaching at least for thirty years, and yet he wants
books!He had seen the Lord, and yet he wants books!
He had had a wider experience than most men, and yet he wants books!
He had been caught up into the third heaven, and had heard things which it was unlawful for a men to utter, yet he wants books!
He had written the major part of the New Testament, and yet he wants
books!
The apostle says to Timothy and so he says to every preacher, "Give thyself unto reading." The man who never reads will never be read; he who never quotes will never be quoted. He who will not use the thoughts of other men's brains, proves that he has no brains of his own.
Brethren, what is true of ministers is true of all our people. You need to read. Renounce as much as you will all light literature, but study as much as possible sound theological works, especially the Puritanic writers, and expositions of the Bible. We are quite persuaded that the very best way for you to be spending your leisure, is to be either reading or praying. You may get much instruction from books which afterwards you may use as a true weapon in your Lord and Master's service. Paul cries, "Bring the books"—join in the cry.