I second the HD shop manual. If you are really into books, the parts manual is an excellent choice as well for the exploded parts views that are most helpful.
After I bought my bike, one of my 1st purchases was the H-D shop manual, the Electronic Diagnosis manual & the H-D parts manual (3-books), but IMHO a Great investment.... by the way I've used all 3 in ordering parts & just studying & learning...