The cornerstone is the most essential element in building a house. It is the foundation that the rest of the house stands on. Even with most skyscrapers, the foundation can and often does extend further below the ground then it does above the ground or at least to the bedrock. It is much like a tree that has roots extended below the earth's surface to keep the winds and storms from toppling it. Like the tree, and the building, if we are not firmly rooted in our foundation, the "storms of life" will blow us away.
Jesus spoke of this happening in Mark 4:7-20. A sower, which is a person who scatters seeds, went to sow the word (seed). The heart is the soil. Some seed falls on the wayside, which are people already skeptical about the word, and Satan comes immediately and takes away the word that was sown in their hearts. Then there are those that are sown on a hard heart, not wanting to change, but then receive it with gladness. But the seed never takes root, and as soon as tribulation or persecution arises (the storms of life) they stumble. There is also seed that is sown among thorns, and as soon as it is sown, the cares of the world, the deceitfulness of riches and desires for other things entering in, choke the word and it becomes unfruitful. In other words, they know the truth, but they compromise the word for the temporary things of this world. We want to be the ones that have soft, fertile, hearts with the word rooting deep and the tree, which is us, bearing good fruit, pleasing to God (Galatians 5:22-23 and Matthew 7:15-23).
We are to lay a cornerstone that is basically, the faith we have in Jesus Christ. Jesus is to be the rock of our foundation (Matthew 7-21-27 and Luke 6:46-49). Before we can press into the deep things of God, we must grasp hold of the elementary principles of Christ; such as repentance of dead works, faith towards God, Baptism, laying on of hands, resurrection of the dead, and eternal judgement (Heb. 6:1). We need this foundation so our walk with Christ will be sturdy and remain intact through trials, tribulations, and persecutions (I Corinthians. 3:5-17).
The contents of the book is to help you lay that foundation, so that you may have a closer relationship with God as He desires (1Peter 2:1-10, Isaiah 28:16, and James 4:8). If you will take these principles and apply them to your life, you will be blessed. But you must be a dooer of the word, not just a hearer (James 1:22-25)