Yes, 7th graders should be able to handle them easily.
Here, from Wikipedia, is the list of books in the order they were published.
1. The Cat Who Could Read Backwards, 1966 (ISBN 0-515-09017-4): Qwill and Koko's first meeting.
2. The Cat Who Ate Danish Modern, 1967 (ISBN 0-515-08712-2): Qwill is given the interior design beat for his newspaper. He also meets Yum Yum.
3. The Cat Who Turned On and Off, 1968 (ISBN 0-515-08794-7): Qwill goes to Junktown to write a feature series.
4. The Cat Who Saw Red, 1986 (ISBN 0-515-09016-6): Qwill gets assigned to the culinary beat and reunites with an old flame.
5. The Cat Who Played Brahms, 1987 (ISBN 0-515-09050-6): Qwill leaves the big city to move to Pickax.
6. The Cat Who Played Post Office, 1988 (ISBN 0-515-09320-3): Someone seems to be trying to get rid of Qwill, and a brightly painted room has his mustache twitching.
7. The Cat Who Had 14 Tales, 1988 (ISBN ?-???-?????-?): A collection of short stories based on the same characters.
8. The Cat Who Knew Shakespeare, 1988 (ISBN 0-515-09582-6): Fire and death at the newspaper offices.
9. The Cat Who Sniffed Glue, 1988 (ISBN 0-515-09954-6): Vandalism appears in Pickax and couple is murdered.
10. The Cat Who Went Underground, 1989 (ISBN 0-515-10123-0): Qwill goes on vacation and carpenters in the area start dying.
11. The Cat Who Talked to Ghosts, 1990 (ISBN 0-515-10265-2): Iris Cobb is found scared to death in her own home.
12. The Cat Who Lived High, 1990 (ISBN 0-515-10566-X): Qwill returns to Junktown to try to restore an old apartment building.
13. The Cat Who Knew a Cardinal, 1991 (ISBN 0-515-10786-7): Pickax's principal is found fatally shot after a party at Qwill's barn.
14. The Cat Who Moved a Mountain, 1991 (ISBN 0-515-10950-9): Qwill and the cats go on vacation.
15. The Cat Who Wasn't There, 1992 (ISBN 0-515-11127-9): Sixteen people from Pickax go on vacation to Scotland.
16. The Cat Who Went into the Closet, 1993 (ISBN 0-515-11332-8): Qwill attempts to solve a mystery long distance.
17. The Cat Who Came to Breakfast, 1994 (ISBN 0-515-11564-9): Qwill takes a trip to Breakfast Island.
18. The Cat Who Blew the Whistle, 1995 (ISBN 0-515-11824-9): A railroad buff and president of a local bank disappears.
19. The Cat Who Said Cheese, 1996 (ISBN 0-515-12027-8): A stranger checks into the New Pickax Hotel. Not too long after, the hotel is bombed.
20. The Cat Who Tailed a Thief, 1997 (ISBN 0-515-12240-8): Small actions of theft are occurring in Pickax and a few people are found dead.
21. The Cat Who Sang for the Birds, 1998 (ISBN 0-515-12463-X): An old barn across from the new art center burns down.
22. The Cat Who Saw Stars, 1999 (ISBN 0-515-12739-6): Qwill takes a vacation in Mooseville.
23. The Cat Who Robbed a Bank, 2000 (ISBN 0-515-12994-1): A jewelry dealer from Chicago comes to visit and is murdered.
24. The Cat Who Smelled a Rat, 2001 (ISBN 0-399-14665-2): A very dry year and concerns about wildfires have everyone praying for snow.
25. The Cat Who Went up the Creek, 2002 (ISBN 0-515-13438-4): Qwill and the cats stay at a riverside inn.
26. Short and Tall Tales: Moose County Legends Collected by James Mackintosh Qwilleran, 2002 (ISBN 0-515-13635-2).
27. The Cat Who Brought Down the House, 2003 (ISBN 0-515-13655-7): An aging film star retires to Pickax.
28. Private Life of the Cat Who..., September, 2004 (ISBN 0-515-13832-0), a collection of pieces from several previous works.
29. The Cat Who Talked Turkey, 2004 (ISBN 0-399-15107-9): The long-absent turkey population begins to return to Pickax
30. The Cat Who Went Bananas, 2005 (ISBN 0-399-15224-5): A new bookstore opens.
31. The Cat Who Dropped a Bombshell, 2006 (ISBN 0-399-15307-1): Moose County prepares for its 150th anniversary.
32. The Cat Who Had 60 Whiskers, 2007 (ISBN 978-0399153907): Polly unexpectedly takes a library job in Paris, leaving without even saying goodbye to Qwill, and a gardener's so-called accidental death does not seem so accidental.
33. The Cat Who Smelled Smoke, 2008 (ISBN 978-0399154744): The book was supposed to be released in April 2008, but it has been postponed.
Other authors have published books related to the series: The Cat Who...Companion (ISBN 0-425-18642-3), The Cat Who... Quiz Book (ISBN 0-425-19187-7), and even a Cat Who... Cookbook (ISBN 0-425-17674-6). A parody of the series known as the The Cat Who Killed Lilian Jackson Braun has also been published.
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