If you just want to use them, reading some good QFT books will serve you just fine. However, even in the best of QFT books, usually a lot of things are swept under the carpet (most notably: some possible infinities in the integration measure).
If you want a mathematically rigorous treatment, I cannot really help you, except to refer you to the ArXiv (www.arxiv.org[/url]). If you use the search form and search for "path integral" in the title and check only the Mathematics box, you might find some interesting articles. For example, in [url=http://arxiv.org/abs/math-ph/0012017v1]math-ph/0012017[/URL] I find
[quote]
Since Feynman’s invention of the path integral, much research
have been done to make the real time Feynman path integral mathematically rigorous (see [6], [9], [10], [13],[18], [19], and [20]).
[/quote]
So you might check out those references, for starters.