SQL & PL/SQL

Can anyone please tell me that where from I can findout the simplest document to understand about Cost Base Optimizer.
Which can define
- What & Why is NESTED LOOPS, INDEX ROWID, BUFFER (SORT), INDEX (SKIP SCAN), MERGE SORT, HASH ** ... ** and many more...

- To understand why Optimizer doing so.
- To Understand, What could be the Best way.
- To Understand, What to do with Query.
- How to Read Execution Plan

Please suggest as simplest as possible.

Thanks.