Comfy furniture and lighting are more important than anything else. If you can't read comfortably, no one will want to be in there. Building your own sheling units can save you some $$ in allowing you to create exactly what you want in a solid wood rather than particle board. Plans can be found at www.buildeazy.com.
For lighting goto thrift stores, antique shops, swap meets, auctions, flea markets, yard and garage sales for great used pieces or www.lightingdirect.com for a great variety of new lights.
I am assuming you have a collection of books already. But if not you can find great deals at the same used sales as I listed above for the lighting. That's where most of my books have come from and I have alot...more than i have room for...of 1st editions and mint condition hardback of my fave authors. I have a yard sale very year to sell off the ones I have read and don't want to save.
you can find furniture the same ways too. Just make sure you are buying it from a family or store that doesn't look gross. You can also tell by the condition of the other items for sale...good stuff or dirty old junk.