lis, here's the deal:

if you want anyone to help solve this problem for you, you're going to have to put down how many books you have. i don't know the best way for you to do this, exactly, maybe you could give an estimate of the total number of books, and then an estimate of the average book width. and then if you have any oversized coffee-table type books, that would change things, too. or you could line up all of your books side by side and measure them with a really long tape measure. but you don't want a square volume measurement, that will just throw you off.
Once you have figured that out, then it should be relatively simple to figure out how many feet of books each shelf can hold, and then how many shelves you need to fit all of your books.

so, that is all. see you later

I've always heard the library rule of thumb as 10 books per square foot for regular books, 6 for bound journals. The "squareness" allows for aisles, etc., but I think you could probably guesstimate based on the linear measure: 10 books per linear foot. Actual mileage will vary of course. My own collection is closer to 12 books per linear foot. Hope this helps.

Interesting. I've looked at my own shelves at home and there are about 50 books (give or take a few) on a linear metre. That calculates to about 15 books/foot.

I do it completely differently: How much space to do you have for bookcases? Buy that many bookcases, upto the amout of money you can affoard. Fill the bookcases with books, if not now, later. Between now and then, fill the shelves with other things, or place particularly interesting covers facing the room.

But I'm a book nerd, and I have never ever ever had enough bookcases.

"Architectural Graphic Standards" gives the following numbers for bookshelf capacity. For each book category, the first number is the number of volumnes per linear foot of shelf, and second number is the recommended shelf depth: Children's Books 10 to 12 (8 in.), Fiction 7 (8 in.), History and General Literature 7 (8 in.), Reference 7 (10 in.), Technical and Scientific 6 (8 in.), Medical 5 (10 in.) , Law 4 to 5 (8 in.), Bound Periodicals 5 (10-12 in.), U.S. Patent sepcs 2 (8 in.). Hope this helps.