I am currently reading the following :

Oracle Database 10g OCP Certification All-In-One Exam Guide (Oracle Database 10g Handbook) (Paperback)

If you are interested in taking the OCA then OCP, then you should certainly take a look at the book.
Oracle Press is known for its pricy books (most are in the hundereds) but this one is pretty affordable.
The book is split in two parts:

Part 1 covers OCA (about 24 chapters).
Part 2 is OCP (around 17 Chapters).

I plan on Blogging interesting pieces of the book here so be on the lookout.

Used start at $58.

Every organization using a database management system (DBMS) to manage data requires a database administration group to ensure the effective use and deployment of the companyâ??s databases. Since most modern organizations of any size use a DBMS, the need for a database administrator (DBA) is greater today than ever before. However, the discipline of database administration is neither well understood nor universally practiced in a coherent and easily replicated manner.

If you are interested in a career as a DBA then you may want to read about it…

this is a great overall description of what you may be getting yourself into:

http://www.dbazine.com/ofinterest/oi-articles/dba-1