That is _ interesting book? an or a ?

that is an interesting book .

to distinguish between a , an  look at these rule 

A +singular noun beginning with aconsonant;

a car   a book ......

An + singular noun beginning with a vowel; 

an apple , an egg.........

an

An interesting book.

An-becuause-I-sonic character

That is an interesting book

The vowel " an " will be used hereThis is an interesting book.

the answer is

That is an  interesting book

First i am not ebglish teacher but i always balance my phrase according to the logic sense , meaning if i a said a then when i spell it , it will be heavy weighted but when i say an then its less weighted by sequence i think the answer is AN.

the answer is "an"

AN because interesting has a Consonant sound

i think an