The [*Encyklopädie der mathematischen Wissenschaften mit Einschluss ihrer Anwendungen*](https://archive.org/details/encyklomath101encyrich) (20+ volumes) was organized from basic to advanced, as was its [French translation](http://gallica.bnf.fr/Search?q=Encyclopédie+des+sciences+mathématiques+pures+et+appliquées). It's a bit dated though.

Nowadays there is also the Russian [*Encyclopaedia of Mathematical Sciences*](http://link.springer.com/bookseries/855) (VINITI, translated by Springer, 80+ volumes).

They contain no proofs nor exercises, so are quite unsuited to studying what you would in a textbook. (Likewise Wikipedia; even Bourbaki is not meant for this.)