"This book is long."
Can long be understood as long to read i.e. has many pages, or is it more literal as in tall or wide?
高(たか)いと細(ほそ)い？ 低 (ひく)いと広( ひろ)い？
And well, when in doubt you can just say: その本の大きさはすごい！
I forget to add です but it still tell me that true, does it still coplete sentense?
Yes. The so-called "い adjectives" can be used independently: ながい means "it is long" (rather than just "long"), so adding です makes it more polite, but isn't required. Grammatically speaking, a form of です is only necessary in sentences without a verb or い adjective.
If a formal sentence ends in an い-adjective followed by です, you can remove the です to make it informal.
長い, as i understand, can be use to describe something as tall, wide, long... in this case, it describe the book is long as in pages. It can be pretty vague to use this sentence. It would be more accurate for the speaker to say "the page number of this book is too much" or "the height of this book is too long"