How many books 25MM wide can fit on a shelf 1 meter wide?

I presume you mean 25 mm (25 millimeters) wide, not 25 mega-meters (25,000 km, which is about twice the diameter of the Earth), and that this is not a trick question about a shelf 1 m wide meaning deep, so the answer depends on what the length of the shelf is. A meter (I'll use my nation's spelling, you use yours!) is 1,000 mm, so we divide 1,000 by 25. The answer is 40.