Kathysierra& BertBates is the best book ever for scjp.
But u can refer Khalid Mugal SCJP book also....
The difference b/w scjp 1.5 &6 is that
Number of questions: 72 (no change)
Time: 210 minutes (increased from 175 minutes)
Passing score: 65% (increased from 59%)
Format of questions: No change.