Hi Ian,
Can't help with tutoring books, but I can say that it would be a pretty fancy trailer if it needed Calculus.
Load increase on hitting ramps etc would be an assessment you make for your design, but generally a 2.5 safety factor is considered a minimum, but it can be anything up from that as long as you can afford the extra metal.
The "decide what you want," is good start advice. Another good start is to find what costs are involved for the things you will not be fabricating, such as axles, wheels, springs etc. They will define the style, size and weight of the trailer and it's load.
No point designing a trailer that requires an axle that is an inch wider than something freely available off-the-shelf for a third the cost. So, I suggest you figure out what you want then look for stuff that will work for you and build the trailer within those confines. If money is no object then design and build anything you like.
Probably the best all round book for reference and formulas is Machinery's Handbook. Generally referred to as the "Bible" in Engineering circles and now in it's (I think) 28th version.