Step 1: Uninstall iBooks and then reinstall it. Don't launch app after it finishes downloading.
Step 2: Launch iFile. You can get it from Cydia if you don't already have it.
Step 3: Copy iBooks.app from here: /var/mobile/Applications/iBooksFolder/ to here: /Applications.
Step 4: Tap /Applications/iBooks.app/info.plist and open it in property list editor. Tap on CFBundleIdentifier and rename com.apple.iBooks to com.apple.iBooksFix
Step 5: Close iFile and respring your iPhone, iPad or iPod touch.
Step 6: After respring there will be two iBooks icons on Home screen. Uninstall the one with rounded corners and keep one that has a square icon.
Do keep in mind that after this procedure you will not be able to purchase books from the iBooks store or transfer your existing purchases from iTunes. You will have to manually sync you books to your device so that they can be read using iBooks app.
How To Manually Transfer Books To iBooks App:
Open iFile and tap the WiFi button on the bottom. Launch any web browser on your computer and enter this address: http://iphone.local:10000/. Then navigate to var/mobile/Documents. Clicking on Choose File will enable you to upload the books you want to iBooks.app, the format for these books is .epub. Simply upload your .epub files from here and then go to the var/mobile/Documents path from iFile on your jailbroken device. Here, tapping on the .epub files that you just uploaded will open the relevant book in the iBooks.app and now iBooks on your jailbroken iOS 5 device will not crash.