She wants convincing proof whether she is obliged to return a book she borrowed, even if that is ten years later
Question

                when i was 10 or 11 im not really sure which age i was, i borrowed a book from my school. I never brought it back and now i am 17. I have still got it but it is a bit ripped. I want to know if i have to return it. Do i have to return it or do I not have to.
Praise be to Allah.
      The Islamic 
      ruling for anyone who borrows something, whether that is a book – as in 
      your question – or anything else, and whether one year has passed or 
      seventy years, is that it is obligatory to render back the trust to those 
      to whom it is due, and to repent from falling short in returning the 
      borrowed item to its owner. We think that rulings such as this are quite 
      clear; in fact this is a given among all people, for several reasons, each 
      of which is sufficient to convince you if you think about it. These 
      reasons include the following: 

    Firstly: 

    The teachings of 
    Islam enjoin that. We have previously quoted a great deal of evidence in 
    fatwa no. 83599. We would add here further clear evidence: it was narrated 
    from al-Hasan, from Samurah ibn Jundub (may Allah be pleased with him) that 
    the Prophet (blessings and peace of Allah be upon him) said: “The borrower 
    owes what he took until he returns it.” Narrated by Imam Ahmad in 
    al-Musnad (33/277). The commentators on the Mu’sasat ar-Risaalah edition 
    said: It is hasan because of corroborating evidence. Adh-Dhahabi said: Its 
    isnaad is good. Al-Muhadhdhab (7/3415). Ibn al-Mulaqqin said: [It is 
    saheeh] according to the conditions of al-Bukhaari. Al-Badr al-Muneer 
    (6/753)

    In ‘Awn 
    al-Ma‘bood (9/344) it says: 

    This hadeeth 
    indicates that the individual is obliged to return that which he took of the 
    property of others, whether that was by borrowing or renting or otherwise. 
    (This obligation remains in effect) until he returns it to its owner. End 
    quote. 

    Secondly: 

    The moral and 
    ethical values on which people, believers and disbelievers, young and old, 
    rich and poor, knowledgeable and ignorant, are agreed dictate fulfilling 
    trusts and returning all things to their owners, whether the property in 
    question is small or great. The one who is careless and devours a little 
    unlawfully will go on to transgress with regard to greater amounts, and he 
    will transgress the limits set by Islam, and will pay no heed to ethics and 
    values. 

    Thirdly: 

    Imagine if 
    everyone who borrowed something never returned it at all. How would the 
    world be in that case? How would people interact with one another when no 
    one could trust anyone else? Undoubtedly is an alarming scenario, if it were 
    to happen – Allah forbid. 

    Conversely, you 
    can imagine a society in which all people render trusts back to those to 
    whom they are due, whether the matter was small or great. What a secure and 
    peaceful life people would live in that case. How many other virtues would 
    this virtue lead to! Undoubtedly that would be an ideal world which all of 
    us would hope for. The Prophet (blessings and peace of Allah be upon him) 
    has told us of some of the virtues of the righteous among the earlier 
    generations, and he told us that one of them said (when they were trapped in 
    a cave after a rock fell and blocked the mouth of the cave, and he and his 
    companions sought help from Allah by virtue of righteous deeds that they had 
    done): “O Allah, I hired a man in return for a measure (faraq) of 
    corn. I offered it to him but he refused to take it. So I sowed that corn 
    and cultivated it, until I had acquired cattle and a herdsman thereby (by 
    means of that investment). Then he came to me and said: O slave of Allah, 
    give me my dues. I said: Go to those cattle and their herdsman, for they are 
    yours. He said: Are you making fun of me? I said: I am not making fun of 
    you; they are yours. O Allah, if You know that I did that seeking thereby 
    Your Countenance, then grant us relief. And they were granted relief.” 

    Narrated by 
    al-Bukhaari (2215). 

    Fourthly: 

    Something else 
    that will convince you that the previous answer is correct is if you were to 
    imagine that all your friends borrowed things from you, each of them 
    borrowing one of your possessions, whether it is clothes or other items or 
    books, then none of them returned what they had borrowed from you on the 
    grounds that a long time had passed, and the borrowed item had suffered some 
    kind of wear and tear. Would you be happy with that? Would you think that 
    that was an appropriate kind of interaction between people? Would you not be 
    upset and annoyed, especially if the borrowed item was something that was 
    dear to you or was somewhat prcious? Perhaps then you will understand that 
    this world cannot be in good order unless people pay heed to morals and 
    ethics, and render trusts back to those to whom they are due. This is an 
    important principle of human co-existence. “Verily! 
    Allah commands that you should render back the trusts to those, to whom they 
    are due; and that when you judge between men, you judge with justice. 
    Verily, how excellent is the teaching which He (Allah) gives you! Truly, 
    Allah is Ever All-Hearer, All-Seer” 
    [an-Nisa’ 4:58]. 

    The idea is quite 
    clear: there is no room for trickery and we should aspire to reach the 
    highest level of honesty with everyone. 

    The Prophet 
    (blessings and peace of Allah be upon him) said: 
    
    “Render back the trust to the one who entrusted it to you, and do not betray 
    the one who betrayed you.”

    Narrated by Abu 
    Dawood in as-Sunan (3534); classed as saheeh by al-Albaani in 
    Saheeh Abi Dawood. 

    And Allah knows 
    best.