Recently I exchanged my motorola mobile phone with someone's I phone 4. But the problem is that I lied him that my mobile's battery will last more than 1 day and truth is that it's battery timing was very poor. I have downloaded a lot of islamic books in his I phone 4. I want to know that reading those books are haraam because I got this I phone by telling lie?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.

It was forbidden to lie about the battery and deceive this person the way you did and you bear a sin for doing so as we have clarified in Fatwa 116433. If this defect was considerable in such a way that if the buyer were to know about it before buying the device, he would not have bought it or its value would have been less or he would have been less inclined to buy it, then you should have clearly explained that defect to him before concluding the transaction.

Since you had withheld this information, then it is incumbent on you to inform the buyer of it now. If he forgives you and accepts the exchange anyway, there is no harm in that; however, if he chooses to undo the exchange and take his device back, you have to accept or you may offer him the difference in value to compensate for the defect. If you cannot reach this person now, you can offer that money (compensation for the defect) in charity on his behalf and dedicate the reward to him.

Ibn Taymiyyah said: "If someone sells an object of deception, what is given in exchange for it is unlawful to him only in the same measure as the value of the deceit. He may offer it in charity if he is not able to return it to the deceived buyer. For instance, if a person deceives a buyer into buying a defective commodity for ten Dirhams and this would have been its price had it been free of defect, and it would have been eight Dirhams with the defect, then if the seller knows the buyer, it is incumbent on him to pay the two Dirhams to the buyer or the buyer may take his money back and cancel the sale. If the seller cannot reach the buyer, he may offer the two Dirhams in charity on his behalf." [Al-Fataawa Al-Kubra]

As for the permissibility of using the device and reading the books you downloaded on it, you bear no sin for that because the transaction is valid. An-Nawawi said: "If a person deceives a buyer into buying a defective item without informing him of the defect, then the transaction is valid, but he (the seller) bears a sin (for the deception)." [Al-Majmoo‘]

Allaah Knows best.