We will be traveling in July by train from Amsterdam to Paris, spend 3 days and then Paris to Calais. Should we book the rail before we leave? What happens if we don't make it to a train at the correct time if we buy in advance? Is there a big price difference if we wait? Thanks for your help!

I find that it is usually cheaper to buy your tickets in Europe. Use the German rail site at bahn.de and work out the schedule. Then, when you get to Amsterdam, buy the tickets a few days prior to leaving.

If you buy from SNCF 30 days in advance you are eligible for discounts of 25% or even more depending upon age of traveler and number of travelers traveling together. Try to get an Amsterdam-Paris one-way fare of 67E or less. It's still easiest to buy tickets once you're in Amsterdam. You can do it at Schhiphol Airport or Amsterdam Central. You won't miss your train if you plan to arrive an hour early to relax over a cup of Dutch coffee. If it happens, the type of fare class you buy determines if you can change time. Even if you have a fare that does not permit changes, politely explaining your error to a ticket sales agent can work if you state that you know you made a mistake and you would appreciate help. The Dutch clearks have helped me in the past when I made errors like this.

My advice:

buy the tickets at Schipohol when youy fly into Amsterdam since that ticket office is one of the easiest to use.

unless you mafe specific seat reservations (are you planning on taking a high-speed Thalys service to Paris from Amsterdamm...if so, then you can get seat reservations at the same time).. if you are using regular Eurocity service between Amsterdam and Paris and don't get seat reservations (you can if you want) then you can use the ticket for ANY train (other than Thalys) at any time of day. And changing to another train, again other than Thalys, does NOT depend on your "fare class."

The tickets will be cheaper if you buy them over there as will the seat reservations.

Two comments here. You make airplane schedules on time, why not trains?
In fact you don't have to go through a long security check.

You should buy your train ticket as soon as you can after arrival. I am assuming second class non smoking. I know from experience that seats in that category are the first to sell out.
There are a few key lines in Europe where 2nd class non smoking sells out on weekends. I got nailed on Zürich - Munich on a Saturday and felt that upping the ante and buying 1st class tickets was necessary.

Thanks for the info and as far as being late for the train, only because we are in experienced with train travel, and not familiar with the city, I don't plan to be late, but it could happen.... Thanks again
Virginia