@simplydex as @DTTA said 1 credit = any 1 book.
Also I would recommend you get a membership on the 14th, that way you get 1 credit when you sign up and another credit on the 15th.
The credit distribution process is not automatic, at some point every 15th of the month someone in Japan wakes up and manually gives out the credit.
If you sign up too late on the 15th then you may miss the credit you would normally get on the 15th.