The mechanism of Agree(ment) has been redefined since Chomsky 1995. Since Chomsky 2000, 2001, it does not rely on Spec-Head agreement anymore. Regarding the relation between the subject and V, if suggest to have a look at Pesetsky & Torrego 2007, who give important references and develop a slightly different, but influential departure from Chomsky's 2000, 2001 definition. Adger 2003 is a good book, but note that his version of agreement is non-standard (as it work up- and downwards).
Chomsky, Noam (2000): “Minimalist inquiries: e framework”. In: Roger Martin & David Michaels & Juan Uriagereka, eds.: Step by Step. Essays on Minimalist Syntax in Honor of Howard Lasnik. Cambridge, MA: MIT Press, 89–156.
Chomsky, Noam (2001): “Derivation by phase”. In: Michael Kenstowicz, ed.: Ken Hale, a life in language. Cambridge, MA: MIT Press.
Adger, David (2003): Core Syntax. A Minimalist Approach. Oxford: Oxford University Press.
Pesetsky, David & Esther Torrego (2007): “Thee syntax of valuation and the interpretability of features”. In: Simin Karimi & Vida Samiian & Wendy K. Wilkins, eds.: Syntactic derivation and interpretation. In honor of Joseph E. Emonds. Amsterdam and Philadelphia: Benjamins, 262–294. doi: 10.1075/la.101.14pes.