Do You See It?

It is always ahead of me yet I can never see it. What is it?

There are a number of books on a shelf. If one book is the 4th from the left and 6th from the right, how many books are on the shelf?

Can you complete the sequence puzzle?

AR TA GE CA LE VI LI _

When I got four, I have it none.
When I got two, I have it some.
When I got none, I have it all.

What is it?

There are people and strange monkeys on this island, and you can not tell who is who (Edit: until you understand what they said - see below). They speak either only the truth or only lies.
Who are the following two guys?
A: B is a lying monkey. I am human.
B: A is telling the truth.

Where do you go to learn how to make ice cream?

ENEMY = 2 * LINK
LINK + 2 = HELL

Then what will be TAXI + HELL? Select from the options below:
a) ROTTEN
b) DANGER
c) HEAVEN
4) MORTAL

Different lights make me strange by changing me into different sizes. What am I?

The person who makes it has no need for it. He who purchases it does not use it. The person who does use it does not know they are. What is it?