Erd problem
 

 Apply all steps of conceptual and logical database design on the following requirements: 

The system should be able to store a customer’s name, address, and contact information and a book’s ISBN, publisher, authors, and title information. A customer should be able to purchase one or many books at a time. The system should store the purchase record of customers. It should give us the information in which shelf the book is lying by storing the shelf’s number, floor, and category information. Books are supplied by suppliers. They are stored in the stock. Number of books are deducted from the stock quantity as they are sold. Additional books are demanded from suppliers by looking at the stock record.

Problem : Is stock a separate entity or a attribute and where should i place stock record.

I guess Book but not sure exactly please i need help.

Re: Erd problem
 

The last sentence in your description would lead me to believe it's a separate entity only because a table for Stock would allow a query to be run against that table to get a list of things that might need to be reordered.

Re: Erd problem
 

Now that is quite a good question.

Does the data about the stock belong (totally) to the book itself? Or is the concept of STOCK a seperate item?

The data could go in a number of places, but how and when is it going to be used. The NUMBER_IN_STOCK is going to be reduced whenever a sale of that item is made, so does it belong in the SALES system? The NUMBER_IN_STOCK is going to be increased whenever a delivery of that item is received, so does it belong in the PURCHASING system?

Is stock control a seperate process / procedure, such that it needs its own processes?

You're the designer, these are design decisions. Make them.