sql tuning

 Hi,

Below is the plan for one of my sql statement and it is running very slow now.So please let me know the steps to troubleshoot this problem.

Execution Plan
----------------------------------------------------------
0 SELECT STATEMENT Optimizer=CHOOSE (Cost=1 Card=1 Bytes=59)
1 0 FILTER
2 1 TABLE ACCESS (FULL) OF 'BOOK' (Cost=1 Card=1 Bytes=59)
3 1 FILTER
4 3 NESTED LOOPS (Cost=6 Card=1 Bytes=82)
5 4 TABLE ACCESS (FULL) OF 'SALES' (Cost=1 Card=5 Bytes=90)
6 4 TABLE ACCESS (BY INDEX ROWID) OF 'BOOK' (Cost=1 Card=1)
7 6 INDEX (UNIQUE SCAN) OF 'PK_BOOK' (UNIQUE)

I am confused since in second line it is showing table full access and 6th line it is showing that index is being used.