The Midrash (Esther Rabbah 10:1) explains that Achashverosh was certain that the reason he was afraid of an assassination attempt was because he had overlooked rewarding somebody. Indeed, they found the incident earlier (Esther 2:21) when Mordechai warned the king about Bigsan and Seresh’s assassination attempt.
M’lo HaOmer points out that it is a miracle that, although Achashverosh wanted to sleep, he asked to be read a record book which would seemingly have the opposite effect. After all, he was the king, and affairs of state, regional power struggles, palace intrigue, and other similar events recorded in such a record would more likely excite the king instead of put him calmly to bed.
Yosef Lekach writes that the entire records book was simply a list of the king’s personal debts.
Ibn Ezra, however, states that the book was a list of vows, and the king was feeling guilty and was under the assumption that his insomnia was a punishment for an unfulfilled vow.
The Malbim writes that Achashverosh was frustrated about what Esther’s request might be. Therefore, he asked for the records book to investigate if Esther’s request might merely be for him to reward someone for a favor done for him.
According to the Vilna Gaon, the verse is discussing two separate books – a chronicle of the nation’s entire history, and records book of the king’s personal enactments. Similarly, the Malbim points out that the big book is a public book, available to all the citizens. The smaller book is private, and only available to the king. As we shall see (be”H), the difference is that one is more likely to be manipulated.