Someone has left ..... books behind.
if u could see the 'books' the answer should be in singular form
males has priority in general discourse and vague situations in most languages.
This is the 21st century. Leave out the gender
SomeONE= singular... Their is plural
'Their' is absolutely wrong. 'His' and 'her' are correct, but gender should be left out . 'His or her' is the most appropriate answer.
Since the sentence says Someone...you do not know their gender at all..ergo their