Describe in detail how the ghost speaks of the Garden of Death and how he explain the verse on the library window.

The Canterville ghost describes the Garden of Death to Virginia as follows:

There the grass grows long and deep, there are the great white stars of the hemlock flower, there the nightingale sings all night long. All night long he sings, and the cold crystal moon looks down, and the yew-tree spreads out its giant arms over the sleepers.

The description of this garden contains many reminders of death. The hemlock flower is poisonous, while the yew tree is a symbol of death because it is associated with Hectate, the Greek goddess of witchcraft and death. Night is also associated with death, and the ghost speaks of the nightingale, a bird that sings at night, as well as the moon shining coldly on this garden.
The ghost explains that the verses on the library window mean that he needs someone sweet, gentle, good, pure, and faith-filled like Virginia to intercede for him so that he can die and go to his final rest. She needs to pray for him to die because he himself has no faith. If she prays hard enough, the angel of death will hear her prayers and be merciful, releasing him to death. The ghost also explains that the forces of hell will come and try to frighten her, but that her purity will win out over them. He says:

"They [the verses] mean," he said, sadly, "that you must weep with me for my sins, because I have no tears, and pray with me for my soul, because I have no faith, and then, if you have always been sweet, and good, and gentle, the angel of death will have mercy on me. You will see fearful shapes in darkness, and wicked voices will whisper in your ear, but they will not harm you, for against the purity of a little child the powers of Hell cannot prevail.

The ghost is tired of wandering the earth and scaring people, and he finds in Virginia a sympathetic person who will help him achieve his goal of finally dying for good.