Studying the Eisenstein cocycle by Sczech, I noticed that to understand its connection with the values at negative integers with zeta functions it is necessary to understand the resuts by Siegel in
Siegel, Carl Ludwig. Über die Fourierschen Koeffizienten von Modulformen. Nachr. Akad. Wiss. Göttingen Math.-Phys. Kl. II 1970 1970 15–56.
Sadly, I don't have access to this paper, and the review in mathscinet is in german; even worst, I do not read german (which I'm slowly learning). Do you know any reference in english, french or spanish, that shows Siegel's results, hopefully with proofs?