What is the meaning of Bronze,Silver & Gold books?

Answer 1

Normaly a color-book is a reference to the color of the cover of a book, like the red book, the blue book, etc. Each of this so named books has a meaning depending on who publish it, like IBM Redbooks (it deliver skills, technical know-how and materials to technical professionals). Red is associated with power, hence the red carpet, an so the name Redbooks = power of knowledge.
Sometimes the cover's color it's related to a special meaning given to that color, like:
Silver, related to the film/photography industry (the name silver derivates from the term silver screen a popular usage as a way of referring to projection screens in general and motion picture projection screens in particular).
Glod, associated to valuables, outstanding and excellence.
Blue, related to a plan or technical drawing usually documenting an architecture or an engineering design (the name blue derivates from blueprint).
Yellow related to standards or advise about some topics, or a listings of aspects related to a topic (the name yellow is often used to denote prevision)

Answer 2

Not sure what you're talking about, but those three metals are increasing in value. My bet would be the bronze is the lowest level or cheapest, with gold at the other end.