The first day of the book, 1/8

The next day, the whole book 1/8+ 12.

On the third day, the whole book 1/8+ 12+6.

I read 3/8+30 of this book in three days.

It is equivalent to 1/2 of the whole book, so 18 page is 1/2-3/8 of the whole book.

The whole book is (12+12+6) ÷ (1/2-3x1/8) = 30 ÷1/8 = 240 (page).