When training assistant clerks I've used the following scriptures to give them insight into the spiritual aspect of their calling:
A Record in Heaven
Malachi 3:13-18 – book of remembrance those thought upon name of Lord
Revelations 3:5 – faithful not blotted out of the book of life
D&C 76:68 – names written in Heaven
D&C 127:7 – record in Heaven
D&C 128:8 – out of books dead judged
Revelations 20:12 – dead judged from book of life
D&C 128:6-8 – judged out of books, books opened, judged by works written; “whatever you record on earth shall be recorded in Heaven”
D&C 128:10,14 – Peter has sealing keys; records for dead are part of sealing power and keys
Moroni 6:4 – numbered among the people of Christ
D&C 132:8 – Behold, mine house is a house of order, saith the Lord God, and not a house of confusion.
Otherwise there isn't a single introduction training for clerks, but the wiki has multiple resources that are useful. For assistant clerks your most useful resource would be
https://tech.lds.org/wiki/Clerks Standard videos for training would be at
http://www.lds.org/pa/display/0,17884,6074-1,00.html
Those cover your main bases but sitting down with the person and doing MLS things together is usually the best way I've found.