Metals and How to Weld Them is a good basic book to start with. There are tons of texts, and bulletins on metallurgy out there. I would start with the Lincoln book, and then check with a local community college on what text their metallurgy class uses and pick up a copy of it also.

https://ssl.lincolnelectric.com/foundation/item.asp?prodnum=MHW&PID=16

Also check with your local AWS Section, they have a complete library available to the members. Lots of good reading in there and it is kept current and up to date.

Linnert's book is good, as well as Metals and How to Weld Them.  Linnert's book may be a bit of overkill for the CWI exam, but the knowledge will always be helpful.  The CWI test, like all other certifications, is just a license to learn anyway!

I have a text book from ASM that is great, but definately overkill for the CWI examination. The book presents the information in such a way that it is "easily" understood. It is actually a Materials Science text book that covers metals and cerramics. It covers the basics and does a good job of covering iron and iron alloys and how the microstructure can transform from one phase to another by heating and cooling that is easily understood. That is one subject the other texts struggle to explain.

I like George Linnerts book. It goes into much more detail than the text book from ASM, but I found it more difficult to put all the pieces together.

I read the book from Lincoln and while it is OK, it isn't one that I would recommend to add to a professional library.

I would list the title of the book from ASM, but I would have to wade through a pile of books waist high and that might take a while.

Best regards - Al