[janeseymore09092]

Salam 'Aleykum,

That's because the Quran and the book of Musa (as) are called Imams:

So is one who [stands] upon a clear evidence from his Lord [like the aforementioned]? And a witness from Him follows it, and before it was the Scripture of Moses an Imam and a mercy. Those [believers in the former revelations] believe in the Qur'an. But whoever disbelieves in it from the [various] factions - the Fire is his promised destination. So be not in doubt about it. Indeed, it is the truth from your Lord, but most of the people do not believe. (11:17)

And before it was the scripture of Moses an Imam and a mercy and this is a confirming Book in an Arabic tongue to warn those who have wronged and as good tidings to the doers of good. (46:12)

[Mention, O Muhammad], the Day We will call forth every people with their Imam then whoever is given his book in his right hand - those will read their records, and injustice will not be done to them, [even] as much as a thread [inside the date seed]. (17:71)

Indeed, it is We who bring the dead to life and record what they have put forth and what they left behind, and all things We have enumerated in a clear Imam. (36:12)

^ So the books are also called Imams and this word means "He/It which is followed", anyone who is followed is an Imam, if the people follow the book (Quran) then the book is called an Imam to the people, if they follow a man in prayer that man is called an Imam of prayer, if they follow a Caliph of Rassul-Allah SAWS who leads them then this man is called an Imam (as in leader).