Hi João,
I had to make a change and it looked like this:
SELECT {Book}.*, {Person}.[First Name], {Person}.[Last Name]
FROM {Book} INNER JOIN {Person} ON {Person}.[BookId] = {Book}.[Id]
WHERE {Person}.[First Name] LIKE '%' + @Author + '%' OR {Person}.[Last Name] LIKE '%' + @Author + '%'
Because the relationship between the tables is that 1 Book can have N People (Authors, translators, coordinators, cover designer, etc.)
That way, the error stopped. I hope I got it right in the construction of tables and relations, but if I'm wrong I'll learn.