How to Buy a Gift Card from a German Store for a Russian Store

According to Apple's page, it uses the following order for song purchases:

• Song credits.
• Gift certificate, prepaid card, or allowance credits.
• Credit cards.

Thus, it would appear that you cannot charge a credit card without using up your credits or gift certificates first.

However, similar to what @Nick said, you could just open up another account, add the credit card to it, and then gift the items you want to your original account.

Update: Based on my own tests of self-gifting, these are my findings:

• Gifts will deduct money from the credit card on your account; they will not use up song credits nor gift card credit. This means that you do not need to open another account, just add it as a credit card and gift it to yourself.
• You cannot send a gift to the same email address as your iTunes account. This can easily be circumvented by sending it to one of your other email accounts.
• The easiest method of gifting to yourself is to select the print option. It will show you a code which you then use to redeem the song. It will allow you to redeem the code on the same account.

Updated 17 Dec 2013.

As reported by Macworld, the iBookstore now allows you to gift books:

Gifting in the iBookstore works identically to Apple’s iTunes and App Store counterparts: Just choose the book you’d like to gift, then tap either the dropdown arrow next to the price (on a Mac) or the Share button (on an iOS device).

More details are in their article, but by-and-large it seems to behave exactly as you would expect.

My original answer:

You can’t; iBook gifting isn’t supported yet.

From Apple’s iBooks FAQs page:

Can I gift a book?

No, you cannot gift a book in the iBookstore at this time.

The phrase “at this time” suggests that it’s coming in the future, but Apple haven’t announced anything yet.