Can I use for iBooks purchase?

You would open the iBooks app, and then click on the iBooks Store. Once you are in the store, your App Store and iTunes Gift Card amount would be available to use to purchase books:

What type of gift card do I have? - Apple Support

Best,

GB

Just redeem it to your account normally. iBooks will use any balance in your account just like the iTunes & App Stores will.

See here: Redeem App Store & iTunes Gift Cards, Apple Music Gift Cards, and content codes - Apple Support