I am interested in taking some of these jobs... and I completely understand the instructions provided.

What I don't understand is how to go about introducing myself once there... do you call ahead and let the manager know? do you just show up?

We are freelancers, what exactly do we call ourselves?

The rest of it, I think I'm good on...

Normally you just show up and ask to speak to the manager. Don't forget to sign into the vendor's log book if they have one. Sometimes your instructions will state to make a pre call and then you will call first.

It all depends on the type of job.
If there is a precall required, you call and ask for the MOD (Manager On Duty) Explain who you are, what company you are from, what you are precalling about and give them the day and time you are planning to come in. You may be checking to make sure the product/ display/ shipper has indeed arrived for you to do what you are supposed to do. Expect to be transferred to receiving if you are looking for something that was shipped. Make sure the day you have planned is acceptable to them. I service a drug store chain weekly, if I would show up on Wednesday, which is their truck day, I would be thrown out on my rear end and not allowed to do any work there that day.
If this is a regular merch job, when you go to the store, go to the service desk and ask for the vendor book. Sign in and tell the person who is there what you need to do. They may give you a visitor badge, if you have a company badge, make sure you are wearing it. They may call that department to let them know you are there and a manager or lead may show up to meet you. Just say, "Hi, I'm Mbrasseau, I'm here to set up/ audit/ inventory/ whatever. If you are representing a company, you would add I'm from ______ . Ask politely for anything you need and clean up after yourself. Thank them for their help.
I hope I am correctly understanding what you are asking and this helps.

That was exactly what I needed to know. Thanks!

Usually you'll have a Letter of Authorization (LOA) in your paperwork. I would just show up, say "I'm here to do X assignment" and hand them the LOA, usually followed by request for access to any material shipped to the store that I was told to request.

That would be nice. I've never had a LOA from a merchandising company, only for audits. Who's doing the right thing ? Is it as an IC or W2? If as a W2 I would sign on today.

I merchandise as well as mystery shop. First thing, sign in and ask for a manager or department manager. Say it's Walmart. You are merchandising something in toys. The manager would just probably would tell you to work with the department manager anyway. You could ask at the desk 'who is working in toys today' because sometimes the department manager will not be there. At a speciality store say, a fabric store, just ask for the manager or assistant manager. If you have a introduction letter, even better. If they require an appointment, that will be in your paperwork.

Some years ago I purchased a book about Mystery Shopping. It was terrific. I still have it and it got me started in mystery shopping. I totally understood how everything worked after I read (and re-read) the book. So now, here's the question: Is there a book that I can purchase/get free or as an ebook that explains Merchandising including how the jobs are set up, what a plan-o-gram is, what are resets, and the easiest way to put together POS displays and seasonal displays.

[www.narms.com]

This link is part of NARMS proficiency training and gives some really good information.( Free) If you chose to take the test it is a breeze. I think it costs $20 now. As for a book, I have never seen or heard of one. Many companies have their own terminology, the way they set up their planograms, and how they want things done specific to their procedures. It really is a learn by doing process. You can learn a huge amount by being part of a large reset crew, they are also more forgiving if you make a mistake. One company has the #1 placement on the top left of the set. Another has the #1 on the bottom left. ( Which I prefer, as it makes more sense to me) Some companies mirror reverse the planogram (Yea!) Others you have to reverse it in your head (Boo)
You will find some companies make their planograms so clear, common sense and reasonably timed, that you can't wait for them to offer something. Then there are the ones that have such miserable, poorly thought out, under timed planograms that you pretend to be someone else when they call.
Seasonal displays can be anything from a 26 item Christmas tree display(-did it, never again, even with a crew of 6) to a shipper full of seeds. POS could be anything on, or near the cash registers. The shippers for Farmers Almanac come to mind, sort of impulse buy items.