Hi, according to your local network configuration, you have to put this in your openvpn.conf :
push "route 192.168.1.0 255.255.255.0"
(I'd advise you to change your local network into something less common like 192.168.XXX.0 to avoid any conflict with possible lans you might be in when connecting to your VPN).
server 192.168.YYY.0 255.255.255.0
(same, choose something unsual for YYY). This will be your VPN route, and your MyBook will become the VPN's router at 192.168.YYY.1.
Note that you'll probably get an error when trying to connect your VPN from your MyBook's Lan (192.168.XXX.0), but anyway you'll have to forward OpenVPN's port (default: 1194) in your router to access it from the internet.
Hope this helps ;)