My apologies if this question is a duplicate. I seached, and the closest I could locate is this question, which has very intriguing and intractable (for me) responses.
In my continuing journey of entering the world of automorphic forms and the Langlands program, I keep finding a particular stumbling block to my understanding: the Eisenstein series and its meromorphic continuation.
As the Eisenstein series is very central to all things automorphic from Langlands-Shahidi, Rankin-Selberg integrals, the spectral decomposition as well as the many arithmetic applications, I find it disturbing when the properties of the series are referred to as a massive black box.
Now, I have read Bump's Automorphic Forms and Representations in which he proves the continuation and functional equation from the properties of the constant term (ie: of Hecke-Tate L-functions), but as I understand it (so... vaguely), this is backwards to the main thrust of Langlands work: Langlands proves the analytic properties of the Eisenstein series without reference to the L-functions, which allows him to port such properties to any L-function one can associate to an Eisenstein series.
For higher rank groups, I understand things are much more complicated (non-constant residual forms, several constant terms, cuspidal Eisenstein series, etc.).
As of now, I have no sense of where the functional equation comes from, nor do I have sense of why a tool, which I understand as global parabolic induction, is so intimately tied to Langlands L-functions.
My hope is that there exists a way to come to an understanding of Eisenstein series (their analytic properties and role) without the extremely daunting task of working through Moeglin-Waldspurger's Spectral Decomposition and Eisenstein series or Langlands' monograph (monolith?) On the Functional Equation satisfied by Eisenstein series.
So this is my question:
Is there an expository reference which deals with the general theory (spectral role and the proof of the meromorphic continuation/functional equation) of Eisenstein series for reductive groups?
If the answer is in the negative, then my follow-up question would be
How do experts in automorphic representations and related fields (I am very interested in different approaches) understand the properties of and role played by Eisenstein series?
Finally, I want to say that my primary interest in this question is the Eisenstein series on reductive groups (or perhaps their covers). However, if there is intuition to be gained by looking at geometric or loop group analogues, I would be very interested.