I am doing an estate sale for a friend. There are lots of marbles. I know nothing about marbles and was wondering if there is a book that is good for all general IDing? I looked on a collector book site and there are lots of books on collecting marbles. I am pretty sure most of the marbles are from the 50's or before. I think there are some that might be valuable, but need to educate myself. Any suggestions? I read some of the posts and am fascinated (and amazed) that there are so many kinds of marbles. Thanks in advance for any info and advice.