Now I am feeling guilty that I didn't buy my kids more from the Scholastic books catalogues.
I usually only allowed my children to pick once book each but some months I couldn't even afford that. This isn't to say that my children didn't have plenty of books but the majority of them were bought secondhand or at book sales. Each year I would take my children to the book sale at the main library. Children's books were about 50c and I would give my children $5 each to spend.
Don't feel guilty. Used book stores are another VERY fond memory of growing up--cost effective, for starters, AND you find all kinds of rare jewels that just aren't in print anymore. And of course there's that special old-book smell that you just don't get anywhere else, one that I learned to love very early in life.

I still have my very first adult-level evolution book that I asked my parents for in 1st grade. They were thrilled to let me have it given the level it meant I could read at! Some of the information is outdated now that new discoveries have been made, but some of the science in it really had a lot of impact even on the fanfic writing I do to this day. (I didn't just pull the term "therapsid" for Cardassians out of thin air...

This is it. Too bad you can't see the illustrations inside, because they are pure awesome.
http://www.amazon.com/Beginning-Story-Human-Evolution/dp/0688094767