Not exactly. The title had two separate uses over time:

• 1. Originally it was used by rulers that had been elected king by the Imperial electors but had not travelled to Rome to be crowned Emperor by the Pope. However, from Maximilian I onwards the elected emperors took the title "emperor" immediately upon their election, and there was no longer a need for a separate title to refer to an elected, but not yet crowned, ruler of the Holy Roman Empire.
• 2. As the Holy Roman Empire had an elected monarch, the son of the current Emperor had no right to succeed their father. Emperors got around this by having their son (or other chosen heir) elected during their life time, to rule along side them as (ofiically, at least) co-emperor. The title King of the Romans was given to the subordinate king, who typically had little-to-no actual powers.

If you're looking for a recommendation for a good book on the HRE I'd suggest "The Holy Roman Empire" by Peter H Wilson.