When I moved to Brooklyn, I brought with me around 750 books. I've always had a book-buying problem, and I don't have a parental storage room to access. The consequence is I have to run around with hundreds of pounds of books whenever I'm moving. Once I tricked some friends into carying them to the fourth floor, I had to figure out a way to store them in a small room in a small apartment.
Having spent all my shelf money on books, my original plan was to find bookshelves on the street or free on Craigslist. I probably could have (this strategy worked with chairs), but having boxes and boxes of books piled unopened in your room makes it hard to feel at home in a new space. What I did find on the street was a set of bed slats in reasonably good condtion.
My roommate Max faced a similar predicament, but we came up with a solution. We went to Ikea and he got another set of bed slats ($10) and we stocked up on the cheapest brackets they have (2 / $1). With our third roommate Will's battery-powered drill (if you don't have a drill, try looking for one on Rentalic, Snapgoods, or Neighborgoods), level, and stud-finder, we screwed the brackets into the wall, laid two slats on each one, and screwed those into the brackets.
WIth this design, you can adjust the brackets onto studs, making life a lot easier. No sawing wood required. And you can put up a whole wall of them in an afternoon, at just over two bucks a pop.