[For Clifford-algebras look in "Algebra". For the analytic theory look in 
"Modular functions".]

1755 P. Alexandrov (ed.): Die Hilbertschen Probleme. Leipzig 1983.

2017 A. Bak: K-theory of forms. Annals of Mathematics Studies 98.
Princeton UP 1981.

6066 Eva Bayer-Fluckiger: Galois cohomology and the trace form.
Jber. DMV 96 (1994), 35-55.

J. Benham/J. Hsia: Spinor equivalence of quadratic forms.
J. Number theory 17 (1983), 337-342.

11448 Keith Brown: Concordant forms. Internet ca. 1997, 4p.

J. Cassels: Rational quadratic forms. Academic Press 1978.

4657 Martin Eichler: Quadratische Formen und orthogonale Gruppen.
Springer 1974.

T. Estermann: On the representations of a number as a sum of squares.
Acta Arithm. 2 (1936), 47-79.

L. Gerstein: Basic quadratic forms. AMS 2008, 260p. $55.
An introduction to the arithmetic theory of quadratic forms.

3614 Emil Grosswald: Representations of integers as sums of squares. 
Springer 1985.

8363 Alexander Hahn/O. Timothy O'Meara: The classical groups and K-theory.
Springer 1989.

25833 Nikolaus Hofreiter: Verallgemeinerung der Sellingschen Reduktionstheorie.
Monatsh. Math. 40 (1933), 393-406.

Y. Kitaoka: Arithmetic of quadratic forms.
Cambridge UP 1993, 250p. 0-521-40475-4. Pds. 35.

1893 Manfred Knebusch/Manfred Kolster: Wittrings. Vieweg 1982.

2019 Manfred Knebusch/Winfried Scharlau: Algebraic theory of quadratic forms. 
Birkhaeuser 1980.

T. Lam: The algebraic theory of quadratic forms. Benjamin 1973.

T. Lam: Introduction to quadratic forms over fields. AMS 2005, 550p.

1777 Falko Lorenz: Quadratische Formen ueber Koerpern. 
SLN Math. 130 (1970).

1763 John Milnor: Algebraic K-theory and quadratic forms.
Inv. Math. 9 (1970), 318-344. [8194]

2053 B. Moroz: On representation of large integers by integral ternary 
positive definite quadratic forms. MPI Math. Bonn 67/1990. [3323]

1888 A. Ogg: Diophantine equations and modular forms.
Bull. AMS 81 (1975), 14-27.

1699 O.T. O'Meara: Introduction to quadratic forms. Springer 1973.

Ken Ono: Euler's concordant forms. Acta Arithm. 78 (1996), 101-123.

A. Rajwade: Squares. Cambridge UP 1993, 300p. 0-521-42668-5 (pb). Pds. 28.

1872 J. Rohlfs: Ueber Summen von zwei Quadraten. 1847 Borho, 75-92.

1596 Winfried Scharlau: Quadratic and hermitian forms. Springer 1985.

2563 Joachim Schwermer: Raeumliche Anschauung und Minima positiv definiter 
quadratischer Formen. Jber. DMV 93 (1991), 49-105.

Daniel Shapiro: Compositions of quadratic forms.
De Gruyter 2000.

Carl Ludwig Siegel: Ueber die analytische Theorie der quadratischen Formen.
Ann. Math. 36 (1935), ...

Alice Silverberg: The Hardy-Littlewood circle method.
Mathematics Essay Univ. Cambridge (1980), 30p.

1698 Ernst Snapper/Robert Troyer: Metric affine geometry. Dover 1971.