Riiiiight. Different story all together
Well firstly, it's a vigenere function, just the key generation is a little bit different so get the basic details down first.
1: A running key uses a set text, at random points in the book.
2: It uses a Vigenere-like function to encrypt the text.
So what you first want to do is try and find out the text used as the running key. Secondly, when using a running key encryption somewhere in the ciphertext is where abouts in this set text there is, otherwise the recipient is screwed for decrypting it. So I would begin by finding out the source of the running key, then testing out some numerical values that you find in the text. If there aren't any, and with it being a vigenere cipher I dont think there will be, you've got to try language analysis.
Think about commonly used letters and words. Vowels are in every word, and most sentences have connectives. Another crucial point is the language the source text is in.
There's a few ideas you can try!