Allaah سبحانه و تعالى also describes the Qur'aan as a "light" (noor). Allaah سبحانه و تعالى says,

"This We have revealed to you a rooh of Our command. You knew not what is the Book
nor what is faith. But We have made it [this Qur'aan] a light (noor) wherewith We guide
whosoever of Our servants We will. And verily you guide [mankind] to the straight path." (Sooratush-Shooraa, 42:52)
Light is the thing that shows the person
the path in front of him. It is by light that he can avoid the harmful objects in his way and follow the way that is most beneficial. However, as Shaykh al-Fawzaan noted, this is different from the light that one physically senses in this world. This is a spiritual light. By this light, one recognises and understands what will benefit him both his mundane and religious spheres. He will recognise truth from falsehood and he will be following
the path that leads to Paradise. [1]
This light leads to the Straight Path and Allaah's grace and mercy. However, that light is only beneficial for the one who follows
it. Allaah سبحانه و تعالى says,


"O mankind! Verily, there has come to you a convincing proof
from your Lord, and We sent down to you a manifest light
[this Qur'aan]. So, as for those who believed in Allaah and held
fast to it [the Qur'aan] [2], He will admit them to His mercy and grace, and guide them to Himself by a Straight Path." (Sooratun-Nisaa', 4:174-175)
From the above it is clear that the Qur'aan is the key to the believers' real and true life. Without the Qur'aan, a person is suffering from a spiritual death and living in darkness. In fact, Allaah سبحانه و تعالى has said in reference to the guidance that He revealed and blesses His followers with,

"It he who was dead and We gave him life and set for him a light whereby he can walk among men like him who is in the darknesses from which he can never come out?" (Sooratul-An'aam, 6:122) [3]
Notes:
[1] Muhaadhiraat fil-'Aqeedah wad-Da'wah (Riyadh: Daar al-Aasimah, 1415 A.H.), p. 292.
[2] An alternative interpretation for "held fast to it," is, "held fast to Him," meaning Allaah سبحانه و تعالى. This alternatice interpretation is more consistent with the principles of Tafseer; however, the interpretation given above is more consistent with the context of the verse. Allaah knows best. In either case, thought, the purport will be the same, as holding fast to Him or to His revelation virtually imply one and the same thing.
[3] How to Approach and Understand the Qur'aan, pp. 12-14