Is there something in the order of a Goedel Escher Bach type book? If you've read it you know what I mean. Something compelling that you have to read a couple of times in order to start to get it, but it's so interesting you can't put it down!

I'm going to answer your title question instead of your body question (which to my mind is completely different): what you're looking for in the title is the Princeton Companion to Mathematics. 


The classic answer to this is Courant and Robbins, What is Mathematics? A bit dated, but certainly worth looking at if you haven't yet. 


Penrose's The Road to Reality covers large portions of mathematical physics. This isn't a textbook, and omits many details, but it is as meaty as GEB. 


Mathematics and its History by John Stillwell This book aims to give a unified picture of Mathematics through it's history. The good things about this book are the extremely beautiful figures, interesting exercises and emphasis on the interplay of Algebra and Geometry. 


Saunders MacLane, Mathematics: Form and Function. Very good overview of undergraduate mathematics, showing interconnections between different areas. As might be expected from one of the inventors of category theory, MacLane defends categories as a foundation for mathematics. 


Mathematics: Its Content, Methods and Meaning is an excellent overview of the full body of mathematics. It is large (3 volumes), but comes in a paperback edition that includes all three. The draw is that it is edited by three wellknown Russian mathematicians (Aleksandrov, Kolmogorov, Lavrentev) who wrote some of the articles and solicited the rest from many other Russian luminaries. It was developed as a compendium able to communicate both the vibrancy as well as the importance of each of the areas of the mathematics so that science ministers in Russia could better understand mathematics as mathematicians do. The translation into English is excellent. The first article, a General View of Mathematics, is highly recommended from a philosophical, historical, and phenomenological point of view. 


Modern Mathematics in the Light of the Fields Medal, which is pretty darn good for all its flaws A Panorama of Pure Mathematics, which looks good but I haven't read 

