[Developers] Moving previous/next book properties from /book/book to /book/written_work

[Bryan Cheung]

https://bugs.freebase.com/browse/DA-573

It's was suggested that the previous/next book properties should  
really belong on /book/written_work instead of /book/book .  I'm going  
to be moving /book/book/sequel and /book/book/preceeding_book  
properties to /book/written_work and updating their keys to /book/ 
written_work/next_in_series and /book/written_work/previous_in_series,  
respectively.  This means queries that query the properties /book/book/ 
sequel and /book/book/preceeding_book will need to be updated.  I'd  
like to move these properties in the next week or so; if anyone needs  
more time to update their queries, please speak up.

Thanks
Bryan