OK so not a book shelf, a large mirror maybe, if it is that easy/central to get to it needs to "disappear". Depending on what state you live in penalties for growing are riduculos even if its just for you, better to spend a weekend and do this right and not be found out. Don't show your bros either, unless they grow their own.
You could stud in and drywall the existing door and create a new smaller door with a mirror on hinges or just a mirror hanging over a small entrance (hang a tarp over the inside to keep light in or create a door that swings in). Then do one large HVAC duct over the mirror and have it split so that it is able to be both an entrance and an exit (not the most efficient, but for cooling a closet it might work). An 400w HPS will prob raise the temp at least 15 degees (prob more) if not vented, so I would say venting is a must. Plus your plants need constant air exchange to grow properly.
Next prob, I doubt you have an outlet in the closet, you don't want random cords running into a wall, too many questions. If there is a outlet in the room which the back side would be facing/near the closet you need to create at least one dedicated outlet (minimal) for inside the closet. I can do basic electrical work and wouldn't feel comfortable advising you on this, its not hard though.
Wish I could have a business just designing and constructing secret rooms, its quite fun. Let me know if you have any more questions