I’m not into the gorey, murder stuff!! I just like the good mysteries. Like Nancy Drew, & Kiki Strike. But I’ve read all those…so… any good suggestions?? And I like long books, not like, 100-pagers. 
Mildred Benson wrote many of the Nancy Drew mysteries; what is lesser-known is that she also wrote many mysteries for girls as the series "The Dana Girls."
The writer who completed the Dana Girls series was Harriet Adams, who also wrote the Nancy Drew mysteries after Mildred Benson.
One pseudonym covers both authors: "Carolyn Keene."
You might want to consider reading The Hardy Boys series, at least the first few, as they were authored by Leslie MacFarlane, who also wrote the first Dana Girls mysteries. "Franklin Dixon" was the pseudonym used for those first Hardy Boys series.
You probably could appreciate Lilian Braun’s light mysteries, beginning with "The Cat Who Could Read Backwards."
Any, for very light comedy mystery, P. G. Wodehouse, such as Carry On, Jeeves; typically, there is a light romance motif.
Further light mystery series for girls:
"The Betty Gordon" series is well-liked; "Betty Gordon at Bramble Farm, or the Mystery of a Nobody," is the first title.
"The Kay Tracey Mystery" series is by Mildred Benson (Nancy Drew author) under the pen name Frances Judd.
Mildred Benson continued her writing career as Mildred Wirt with the "Penny Parker Mystery Stories," 17 volumes.
"Mildred Wirt" also authored the "Trailer Stories for Girls" adventure series.
Alice B. Emerson, Mildred Benson’s nom de plume, authored "The Ruth Fielding Series."
Mildred, writing under the name Joan Clark, wrote the "Penny Nichols Mystery Series."
Ann Wirt, another Mildred Benson name, wrote "The Madge Sterling Series."
Mildred Wirt returned to write the "Ruth Darrow Flying Stories" adventure series, and also the seven volume set of individual books, beginning with The Twin Ring Mystery, then Mystery of the Laughing Mask, Sky Racers, Carolina Castle, Courageous Wings, Linda, and Pirate Brig.
Julia K. Duncan, aka Mildred Benson, wrote The Doris Force Mystery Stories (4 vol.).
Mildred also wrote three stand-alone novels, Dangerous Deadline, Quarry Ghost, and Kristie at College; and, as Joan Clark, Connie Carl at Rainbow Ranch, and, as Carolyn Keene, Mystery at the Lookout.
As Mildred Wirt, she authored the Brownie Scouts series, which are worthwhile for a modern 13-year-old, as the characters are more mature than today’s Brownies.
You might appreciate the "Chalet School Series," by Elinor M. Brent-Dyer, which is very popular in England and unknown in the U.S. http://www.the-cbb.co.uk is a site devoted to it; "Register" button permits you to register, and to discuss with others these books.
The "Melody Lane Mystery Stories," Lilian Garis, has nine volumes.
"Mexican Mystery Stories for Girls," a three-volume set, Helen Randolph, chronicles a kinder, gentler Mexico.
The "Mary Lou Series," by Edith Lavell, is interesting in that its teenage girl detective sometimes skirts the law while solving a case.
"The Barbara Ann Series," by Ruth Grosby.
Clair Blank authored at least two series, "The Adventure Girls Series" and "The Beverly Gray Mystery Stories."
"The Arden Blake Mystery Stories," by Cleo Garis.
"The Connie Blair Mystery Stories," by Betsy Cavanna under the pseudonym Betty Allen.
Margaret Sutton wrote the great "Judy Bolton Mystery Stories," which feature Judy growing up as the series continues.
"The Blythe Girls Series," by Laura Lee Hope (pseudonym for Elizabeth Ward).
"The Trixie Belden Series," by Julie Tatham; the character’s age is 13, and then 14.
"The Girl Scouts Mystery Series," by Virginia Fairfax.
"The Billie Bradley Series," Janet Wheeler.
Would also note the recent "Hope Rising," by Kim Meeder, which is a non-fiction account of her solving and healing psychological mysteries and mysteries of horse abuse.
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