The Foundation Trilogy by Isaac Asimov
In middle school - or junior high school as it was known back then - I got into science fiction books after stumbling upon The Source of Magic by Piers Anthony. After that, I was all about big sci-fi and fantasy books, and it was easy to get a whole stack of 'em from the Science Fiction Book Club. One of the first books I got was The Foundation Trilogy by Asimov and I clearly remember showing it off - proud that I was carrying around such a weighty tome as my 'casual reading.' One young lady looked at it and said, "The Foundation of Triology? You read math books?" And faced with the options of agreeing or unwinding it to explain that it was a compilation of the classic works of Asimov and what they were about, I just said, "Yes." She went away somewhere between Impressed and What a Dork. - the usual.
Science Fiction Book Club editions of Isaac Asimov's Foundation Trilogy are going for as much as $90 online... go check your stacks!