The Joan Wilder Effect by Lori Wilde

Straightlaced NCIS agent Joel Hunter always plays by the rules, and he’s been given an important assignment–get friendly with the suspected spy next door, gain her trust, and under no circumstances let her know who he really is.
There’s only one hitch to winning over mousy cartoonist Marlie Montague—this shut-in’s more timid than Joan Wilder behind that door and won’t give any stranger the time of day, no matter how handsome.
Until a pistol-packin’ UPS imposter makes a special delivery: one bullet to make this Marlie’s last cartoon caper. Adrenaline blasts Marlie out of her cautious cocoon and sends her bursting through Joel’s window in a panic, straight into the hunk’s shocked arms. Kaboom! Kablam! Kapow!
Now they’re on the run rogue-style, road-tripping in Joel’s muscle car, throttle punched, driving dirty with death in the rearview. With lives on the line, it’s no time to play things safe in a conspiracy deeper than the Deep State.

Can by-the-book Joel keep his cool with this shut-in turned wild woman riding shotgun? More surprises than a ticking bomb, she leaves him shaken, stirred and chasing after his runaway heart!