Rookie contracts were discussed in full in Nugget 8. This nugget deals with the fifth year option available to teams for players drafted in the first round of any draft.
The book Crunching Numbers deals with the 5th year option in Chapter 8:
FIRST ROUND DRAFT SELECTION
A player selected in the first-round of the draft will have a fixed and unalterable contract length of four years, with the opportunity to earn a fifth-year (team) option attached to their contract.
Fifth year option exercised by May
This fifth-year option must be exercised by the first week in May, in the year following that player’s third contract season.