That Man Callahan by Catherine Spencer (harlequin paperback) 1996
Isobel Whitelaw is cool, calm, professional--and determined to keep Mike Callahan at arm's length!
"From the moment he marched into my office, demanding and ruthless, I knew that man Callahan was trouble! Of course, he had every right to defend his nephew, but not at the expense of my peace of mind. Then he started to invade every corner of my life. And he just wouldn't take no for an answer. Having been burned before, I thought I was immune to love, desire and the whole romantic package. But determination must be Mike Callahan's middle name...!"