“The truth is that even big collections of ordinary books distort space, as can readily be proved by anyone who has been around a really old-fashioned secondhand bookshop, one that looks as though they were designed by M. Escher on a bad day and has more stairways than storeys and those rows of shelves which end in little doors that are surely too small for a full-sized human to enter. The relevant equation is: Knowledge = power = energy = matter = mass; a good bookshop is just a genteel Black Hole that knows how to read”
Terry Pratchett, Guards Guards
Travel Between The Pages 
I somehow missed that quote when reading this book. Makes me think I should reread it.
Just reading Rob Wilkins biography of Terry Pratchett and this quote appears there too, in celebration of Terry’s formative visits to second-hand book shops. Wilkins was Terry’s longtime PA, and was entrusted with finishing the autobiography Terry started when he was diagnosed with Alzheimer’s but which he sadly didn’t live to complete.