The nineteenth century saw a great rise in interest in history, which was probably a Good Thing.
On the other hand, it also saw a lot of slipshod, nationally-biased work where authors either made shit up, or were very careful with what they selected.
Nineteenth century historians were very big on ideas like the March of History towards Progress, and thus tended to promulgate ideas like the Renaissance.
Generally, if it's a nineteenth century work, dont use it ... unless (a) you really know what you are doing, or (b) it's a nineteenth century reprint of a historical document.
While it is not unknown for historical documents to have been fabricated in the 19thC, you are more likely to be on solid ground if you use 19th C document collections (eg the Historical Manuscripts Commission collecteions) than if you use nineteenth century interpretations (eg Burkhardt).
Contributed by Anton, 6/11/03. Feel free to correct, redistribute etc