I still think that in the present context "infinite dimensional linear algebra" = "linear functional analysis" = "large chunk of Dunford & Schwarz". To be slightly less flippant: if someone asks for "some introductory literature focused on such infinite systems of linear equations in infinitely many unknowns over C" then I am sorely tempted to say "go and read all the classical stuff on the Fredholm alternative, then the spectral theorem for normal operators on Hilbert space, then some of the operator theory on other Banach spaces, then look up K\"othe spaces ..." because without all this, I don't know what can be said in general. This also seems to be what an (under)graduate supervisor would be for.
On the other hand, if the question is "here is a system of linear equations in infinitely many unknowns, where I have this a priori knowledge about the coefficients of the equations, then do I have some reasonable criteria for inverting the corresponding linear operator", then that seems more like a question which can be answered well.
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