There must be a proof in Cassels' Lectures on elliptic curves (Cambridge University Press, Cambridge, 1991).
Se also his masterly survey Diophantine equations with special reference to elliptic curves (J. London Math. Soc. 41 (1966) 193–291) and the historical essay Mordell's finite basis theorem revisited (Math. Proc. Cambridge Philos. Soc. 100 (1986), no. 1, 31–41).
Here is a quote from this last paper :
Weil's generalization of Mordell's theorem (and subsequent generalizations) was usually referred to as the Mordell-Weil Theorem. Mordell himself strongly disapproved of this usage and frequently insisted (in public and in private) that what he had proved should be called Mordell's Theorem and that everything else could, for his part, be called simply Weil's Theorem.
Addendum. Another excellent source is Knapp's Elliptic curves (Princeton University Press, Princeton, 1992) which contains a proof of Moredell's Mordell's theorem (over $\mathbf Q$).
There is a very affordable book by Milne (Elliptic curves, BookSurge Publishers, Charleston, 2006) and a very motivating one by Koblitz (Introduction to elliptic curves and modular forms, Springer, New York, 1993). Tate's Haverford Lectures also served as the basis for Husemoller (Elliptic curves, Springer, New York, 2004).