The Importance of Being Married by Gemma Townley
Apparently, this is the first novel in a planned trilogy, and features Jessica Wild, a British gal who told one white lie which spiraled and spiraled...
Jessica befriends Grace, an aging widow who comes to see Jess as an adopted granddaughter, who wants nothing more than for her to find a good man and settle down. To please Grace, Jess "invents" a boyfriend, then fiancee, and then gets "married".
When Grace dies and the will opened, her entire (secret) hidden fortune is left to Jess - in her "married" name. Which gives Jess 50 days to find and marry her invented husband.
This features some great supporting characters (Helen and Ivana were my favorite), despite some outrageous situations and a slightly too-tidy ending, this is a fun and breezy novel for the chick lit crowd.