I want to do a thorough investigation of Hilbert's Theorem and them Efimov's Theorem:
(Hilbert's Theorem) A complete surface $S$ with constant negative curvature can't be isometrically immersed in $R^3$.
(Efimov's Theorem) A complete surface with Gauss curvature $K\leq k_0<0$ can't be isometrically immersed in $R^3$.
and I would like to know if anyone knows good bibliographical references on it apart from the book by Do Carmo. I would really appreciate it.