The original publication and reading order of the Jill books is:

1 - Jill’s Gymkhana (1949)

2 - A Stable for Jill (1951)

3 - Jill Has Two Ponies (1952)

4 - Jill Enjoys Her Ponies (1954)

5 - Jill’s Riding Club (1956)
6 - Rosettes for Jill (1957)
7 - Jill and the Perfect Pony (1959)
8 - Pony Jobs for Jill (1960)

9 - Jill’s Pony Trek (1962)

The Knight paperbacks have altered this reading order.

1 - Jill’s Gymkhana

2 - A Stable for Jill

3 - Jill Has Two Ponies
4 - Jill Enjoys Her Ponies/Jlil and the Runaway

5 - Jill’s Riding Club
6 - Challenges/Pony Jobs for Jill
7 - Jill and the Perfect Pony
8 - Rosettes for Jill

9 - Jill’s Pony Trek

Rosettes and Pony Jobs/Challenges change places.  Why Pony Jobs should be read at number six is a mystery:  it is very obviously the last chronologically, with Jill and Anne both having left school!  

The pictures to the right show the reading order as printed on the original hardbacks (my copies of Stable and Two don’t have dustjackets and obviously Jill’s Gymkhana in its first edition state isn’t numbered.

The set of paperbacks is (mostly) the Black Horseshoe Knight printing, but all Knight printings, as far as I know, kept the same printing order.