Reference for Unitary Group attached to $E/k$

For discussions on forms of classical groups you can look at:

-- André Weil, Algebras with involutions and the classical groups, J. Indian Math. Soc. 24 (1961), 589-623 (also in Oeuvres Complètes).

-- Platonov-Rapinchuk, Algebraic groups and number theory, Ac. Press, 1994.

-- The book of involutions, AMS Coll. Publ., vol. 44, 1998.

-- Kneser, Lecture on Galois cohomology of classical groups, Tata Inst. of Fund. Research, Bombay, 1969.

I've written a summary of Weil's theorems and proofs (following Platonov-Rapinchuk). It is in French ... and only deals with local base fields.