I created a small guide here how to configure the MyBook as an ATA over Ethernet device.
I bought the device to use it as a backup for my RAIDed NFS and Samba share on my home server. I quickly discovered that I did not like the inbuilt functionality, The web inteface did not allow me to define a user name shorter than FIVE letters. In addition, all shares and user names were created with capital letters. This led to very annoying problems when I tried to access the shares from my linux server.
I then tried to search for if someone had managed to use the device as an iSCSI target, but I did not find anything. Fortunately, somebody had already compiled vblade, a similar, but instead of iSCSI, ATA over Ethernet target software, for the NSLU2 (I think). It was only a matter of configuring the MyBook (duh I hate this name…).
The only drawback is the incredibly slow network speed. It seems to be only about 2 MB/s with AoE. Do you have any speed measurements with different protocols?