Let "p" be the price of one book.


Then the "conditional" price ("if-price") in other scenario would be (p+5) dollars.


The "if" equation then is


     -  = 10  books.


Multiply both sides by p*(p+5); then simplify


    300*(p+5) - 300p = 10p*(p+5)

    300p + 1500 - 300p = 10p^2 + 50p

    10p^2 + 50p - 1500 = 0

    p^2 + 5p - 150 = 0

    (p-10)*(p+15) = 0.


Only positive root p = 10 is the solution.


CHECK.   -  = 30 - 20 = 10 books.    ! Precisely correct !


ANSWER.  The price of one (of each) book is 10 dollars.