Finding out the size of an object by comparing images?

Hi everyone,

how to measure an object in a prefixed environment with permanent 
changes of camera focus?

I want to measure books, lying on their back, height and length. At the 
moment, I have one ruler on its right and another on its bottom, at the 
moment I have to enter the sizes manually into the db and to cut out the 
book manually. I already get the black cut out automatically, but still 
have to cut out the rulers from the book.

Now - via image comparison would there be a way to tell its size, that I 
do not have to manually enter that into the db? After all, I do not need 
overall exact measurement. Would a pattern help? Like, if I have no more 
then 30 cm and each cm has a different pattern, one would search the 
first one found, that would tell me what size I have. One should 
remember, that these patterns might slightly change due to different 
light, Is this idea worth following at all or rather a waste of time?

George