The Encyklopädie der mathematischen Wissenschaften mit Einschluss ihrer Anwendungen (20+ volumes) was organized from basic to advanced, as was its French translation. It's a bit dated though.
Nowadays there is also the Russian Encyclopaedia of Mathematical Sciences (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.)