Iqzas,
I won't recommend usage of a certification guide as a reference book. Java 7 includes many language enhancements that are neither covered by the 'OCA Java SE 7 Programmer' or 'OCP Java SE 7 Programmer' certification. You can access the complete list
here.
For an example, new features in Swing aren't covered by any of the above mentioned certifications. It wouldn't make sense for a certification book to cover topics that aren't covered by the relevant exam.
Also, a certification book will define techniques, approach, tips, sample questions etc. to prepare you for the exam, which are not needed in a reference book.
With respect,
Mala