Reduce initial cap on skills to Int / 5 (it was 1 + Int / 5).
Make Item_type_book's time_to_read a function - return value is based on skill cap (if any) and int required - rather than a value loaded from the data file.  Hand-writing time_to_read is useless work and could result in weird values.
Add int chapters to Item_type_book.  Only applies to non-skill books.  It represents how many times we can read it before it's "used up."
Several new Item_groups just for books; add books to some existing Item_groups.
Finish making books for all the mental skills, and add several just-for-fun books.
Add a chance to drop some books.
Load chapters count from data.