Assuming you are a consultant to this company, you want to sell them a non-exclusive license. To your plain-English description, I would explicitly add that you require the company to contract individually for any further projects using your library. Of course, I Am Not A Lawyer (IANAL); you should consult a real one if you are going to write your own contract, especially since your actual laws may vary...
If you are an employee for this company, rather than a contractor, be aware that code written as an employee is usually, by default, work made for hire, which means the company owns it from the get go. If I were working in any environment that even vaguely resembled employment, I would be inclined to a very careful, explicit, and particular written contract, to ensure that the company did not appropriate my library altogether. But, again, IANAL.
Additionally: at least as important as getting the contract right is making sure the people at the company you are contracting with have the same understanding about what they're buying as you do.
The contract is insurance against lawsuits, but mutual understanding is what should make a lawsuit less likely. After all, your question indicates that you expect repeat business...