The Dreadful Lemon Sky

The Dreadful Lemon Sky (1975) is the sixteenth novel in the Travis McGee series by John D. MacDonald. It is the 87th novel in The Top 100 Crime Novels of All Time as compiled by the Mystery Writers of America (it is not included in the U.K. version of the list).
Plot.
Carrie, an old friend of the hero, Travis McGee, arrives at his houseboat, the "Busted Flush". She has a suitcase full of suspicious money. Carrie asks Travis to keep it safe for her for two weeks and to send it to her sister if she does not return in two weeks. For his troubles, McGee can keep $10,000 worth.
A fortnight passes and Carrie doesn't return. McGee investigates and learns she died in a traffic accident but he believes she has been murdered. His investigation leads him to an apartment complex full of singles presided over by a landlord called 'Big Daddy'. He also meets the newly widowed 'Cindy Birdsong', who becomes his new romantic interest.