I greatly enjoyed PJ Evans' article on pages 48 - 51, "Build your own NAS". Setting up a NAS was one of the first long term activities I had used a Raspberry Pi for and with much the same methods as outlined in the article. I still have a RPi 3 running as a NAS, though with a very different arrangement now.
A) SFTP via the file manager would be great for a beginner level follow up article. It is mentioned in Step 13 of the article. SFTP support is built into the file default manager, PCManFM, and is very convenient for file transfers from one Raspberry Pi to another. If OpenSSH is active on the other Raspberry Pi, then the client Pi just needs to open PCManFM and enter the URI in the location bar at the top of the window:
B) An advanced follow up would be to set up the NAS using FreeBSD much like in the article, but with OpenZFS and RAIDZ instead of RAID-1