Hi, David. This is roughly Robin's comment, just with longer discussions, and, I am afraid, links to my own answers. That's life. 

I think you would probably enjoy J.H. Conway, "The Sensual Quadratic Form," especially PSL_2(Z) on pages 27-33.

I always like  "The Markoff and Lagrange Spectra" by Thomas W. Cusick and Mary E. Flahive and 
 "Binary Quadratic Forms" by Duncan A. Buell. 

On particular issues I think you are raising, answers by me and by Dror in:

Reasons for switching from simple continued fractions for $\sqrt D$ to reduced forms:
http://mathoverflow.net/questions/22811/ 

Numbers (here primes) occurring as "diagonal" coefficients:
http://mathoverflow.net/questions/29850/ 

Other:
http://mathoverflow.net/questions/22552/ 

In my language, I think you are asking mostly about small numbers occurring as diagonal coefficients, just not necessarily prime. For me, such numbers need also to be *primitively* represented by the forms in the cycle (for you I guess it is by $ x^2 - D y^2  ,$ using
$a_0 = \lfloor \sqrt D \rfloor$ your quadratic form is equivalent to the reduced form $\langle 1,\; 2 a_0, \; a_0^2 - D \rangle.$  )

phoebus:~/Cplusplus> ./Pell

Input n for Pell

67

0  form   1 16 -3   delta  -5

1  form   -3 14 6   delta  2

2  form   6 10 -7   delta  -1

3  form   -7 4 9   delta  1

4  form   9 14 -2   delta  -7

5  form   -2 14 9   delta  1

6  form   9 4 -7   delta  -1

7  form   -7 10 6   delta  2

8  form   6 14 -3   delta  -5

9  form   -3 16 1   delta  16

10  form   1 16 -3

 disc   268

Automorph, written on right of Gram matrix:

-1106  -17901

-5967  -96578


 Pell automorph

-48842  -399789

-5967  -48842

Pell unit
-48842^2 - 67 * -5967^2 = 1

=========================================




Anyway, let me know if you want to see any more worked examples.