I'm tasked with tracking our MQLs, SALs, SQLs etc. I'm trying to find a good course or book that explains what each should mean and what are the best ways to track these in Salesforce. Any suggestions please?
Firstly, you don't have to name your stages MQL, SAL, SQL
Secondly you don't have to use them all
Really what you should do is map out your sales process or customer lifecycle journey. Think about what key milestones are important to your business and set up your model accordingly, things are going to be different depending if you're B2B/B2C, the industry/vertical or length of sales cycle. There are plenty of guides on the web talking about what stages to consider (and the definitions of those stages) as well as how to build a program/model in Marketo, just google Marketo + RCM or Marketo + Lead lifecycle
Everyone's going to have different opinions. My definitions for the 3 you listed:
MQL - These are leads that have gone through automated filters determining they have met marketing criteria (you do need to go through a thought process on what are your own marketing criteria)
SAL - These are leads that have been handed over to Sales
SQL - These are leads that, after being contacted by Sales, have been determined to be a good fit to do business with
Using the numbers that pass through each stage in a period of time, you can construct a funnel to identify where your processes can be improved
The difficulty with these very widely spread acronyms is that the definition of "Qualified" is multifaceted: when you talk to various people, you get a different definition each time... Plus the fact that the A in MAL / SAL stands sometimes for "assigned", or for "accepted". The last drawbacks with the MQL/SAL/SQL acronyms is that the next step is not straightforward.
This is why we prefer and usually implement: