These are dark days for indie-minded Park Slope bibliophiles. 7th Ave. Books closed a few months ago, and just last week the Times reported that the Community Bookstore had fallen on hard times. Now comes word that another 7th Avenue bookseller is packing it in: Park Slope Books is consolidating operations with its sister store on Montague Street, Heights Books, and closing its Slope location this spring. Sad news for Slopers who like having options beyond Barnes & Noble. GMAP