I am looking for any reference dealing explicitly with Eisenstein series on Siegel space (the simplest case of $\rm{SP}_4$ is fine). Anything would be welcome, but in particular I'm interested in the Fourier-expansions and constant terms. Another preference is that reference be written in the language of the reals instead of the Adeles, though this isn't so important.
Thanks!
UPDATE: Thanks for the references so far! I should have specified, I am interested mostly in the real analytic Eisenstein series, which are induced from the Borel.