The Bible, God's Word

"Within this wondrous volume lies
The mystery of mysteries;
Happiest they of the human race
To who their God has given grace
To read, to fear, to hope, to pray,
To lift the latch, to find the way;
And better had they never been born
Who read to doubt, or read to scorn."
— Sir Walter Scott

Most people would consider this a no-brainer, but most people don't read God's Word, Christian or non-Christian. So this is a must-read to get the whole picture for all the other things I recommend for knowledge and wisdom.