since you are using amsbook, the "sink" (the vertical distance) above the chapter title can be decreased by redefining the value of \topskip in the code that sets the title block.
here is the code from amsbook.cls, with the part to be changed indicated.
\makeatletter
\def\@makechapterhead#1{%
%% change the value of \topskip from 7.5pc to the desired value
\global\topskip 7.5pc\relax
\begingroup
\fontsize{\@xivpt}{18}\bfseries\centering
\ifnum\c@secnumdepth>\m@ne
\leavevmode \hskip-\leftskip
\rlap{\vbox to\z@{\vss
\centerline{\normalsize\mdseries
\uppercase\@xp{\chaptername}\enspace\thechapter}
\vskip 3pc}}\hskip\leftskip\fi
#1\par \endgroup
\skip@34\p@ \advance\skip@-\normalbaselineskip
\vskip\skip@ }
\def\@makeschapterhead#1{%
%% change the value of \topskip from 7.5pc to the desired value
\global\topskip 7.5pc\relax
\begingroup
\fontsize{\@xivpt}{18}\bfseries\centering
#1\par \endgroup
\skip@34\p@ \advance\skip@-\normalbaselineskip
\vskip\skip@ }
\makeatother
\@makechapterhead formats "regular" chapter headings; \@makeschapterhead does the same for the starred version. \topskip specifies the sink to the main title, so it should not be reduced to zero; enough space must remain to accommodate the "chapter n" line.
\documentclass{...}and ending with\end{document}. – Anke Apr 10 '13 at 08:20