Hmm....Not sure what you mean by 'leisurely'. But I'd suggest go read Hardy's Apology if you haven't read it. (Snow's foreword actually made me cry...). You can also watch the seven lectures Feynman gave at Cornell. I already watched them like 10 times..
Hmmm, as much as I respect Papa Rudin, I'd recommend the Princeton Companion to Mathematics, or Mathematics from the Birth of Numbers for more casual reading. Any of the volumes of Dover's The World of Mathematics are also very good. Slightly thinner books I'd suggest are Gleick's Genius book about Feynman, Thirty Years that Shook Physics by Gamow, and maybe Kidder's Soul of A New Machine.
What do you mean by leisurely? Studying at your own pace, or recreational reading?
One of my favorite math books is Fermat's Enigma, by Simon Singh. It's very accessible, compelling, and educational. It covers a lot of ground in mathematical history as it tells the story of how mathematicians over the years chipped away at the problem of Fermat's Last Theorem, until Andrew Wiles finally came up with a complete proof.