noun
BAD : Her books deal mainly with personnel relationships, especially marital problems.
GOOD : Her books deal mainly with personal relationships, especially marital problems.
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personal (adjective) = concerning or belonging to one person in particular; individual or private: 'The novel is based on the author's own personal experience.' 'Our bank manager assured us that she would give the matter her personal attention.'
personnel (noun) = all the people employed in a company, office etc: 'In the event of a fire, all personnel must report to the reception area.' 'The personnel officer wants you to call and arrange an interview.'