I just copied this off a page, the books are in order of publication date, but at the bottom it tells you in what order you should read them.
1) Enchanted Land
October 1978
2) Black Lyon
October 1980
3) Velvet Promise
January 1981
4) Casa Grande
August 1982
5) Highland Velvet
May 1982
6) Velvet Song
February 1983
7) Sweetbriar
March 1983
8) Velvet Angel
December 1983
9) Counterfeit Lady
December 1984
10) Twin of Fire
April 1985
11) Twin of Ice
May 1985
12) Lost Lady
December 1985
13) Temptress, The
August 1986
14) River Lady
December 1986
15) Princess, The
October 1987
16) Raider, The
February 1988
17) Awakening, The
May 1988
18) Maiden, The
September 1988
19) Taming, The
April 1989
20) Wishes
October 1989
21) Knight in Shining Armor
May 1990
22) Mountain Laurel
July 1990
23) Conquest, The
January 1991
24) Eternity
January 1992
25) Duchess, The
July 1992
26) Sweet Liar
August 1992
27) Invitation, The
March 1993
28) Remembrance
October 1994
29) Holiday of Love
“Change of Heart”
November 1994
30) Heiress, The
November 1995
31) Legend
October 1996
32) Gifts of Love
“Just Curious”
October 1996
33) Angel for Emily
January 1997
34) Upon a Midnight Clear
“The Teacher”
November 1997
35) Simple Gifts
”Just Curious”
“Change of Heart”
January 1998
36) Blessing, The
October 1998
37) High Tide
November 1999
38) Temptation
October 2000
39) Season in the Highlands
“Unfinished Business”
December 2000
40) Summerhouse, The
May 2001
41) Mulberry Tree
June 2002
42) Forever
October 2002
43) Wild Orchids
April 2003
44) Forever and Always
September 2003
45) Holly
November 2003
46) Always
November 2004
47) First Impressions
October 2005
48) Caroline Isle
January 2006
49) Someone to Love
July 2007
50) Secrets
May 2008
51) Return to Summerhouse
July 2008
52) Lavender Morning
March 2009
53) Promises
August 2009 (novella e-book & video – vook)
54) Days of Gold
December 2009
55) Scarlet Nights
August 2010
56) The Scent of Jasmine
December 2010 (e-book only)
Reading Order:
1 2 4 3 5 6 8 7 10 11 9 12 14 13 15 16 17 18 19 20 21 22 23 24 25
26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 51 41 42 44 46 43 45
47 48 49 50 52 53 54 55 56