In connection with this topic, I've been asked what my opinion is of David Bick's Metal mines of Wales and Old copper mines of Snowdonia.
First of all, I have to admit a personal bias in that I knew David through the Welsh Mines Society and I had a high regard for him. Also, I don't claim to know a great deal about the subjects of the books concerned, so I can only answer in general terms. That said, my opinion is that David's work, whilst somewhat out of date in a few places, is as as good a general guide/overview as can be found anywhere.
However, it's perhaps worth looking at the sources of error that can be found in any work of this sort.
Mistakes
These happen to everyone, they include misspellings, incorrect grid references and the general ability of people to get things wrong. Unfortunately, once something gets into print it tends to propagate (see below).
Bias
This does not have to be intentional. In connection with a project of my own I noticed that a previous author had given what I considered undue prominence to the narrow-gauge tramways involved in some of the operations. I discovered later that the author was an industrial railway enthusiast - which to my mind explained his emphasis. It's not that such an emphasis is incorrect, but it does mean that different authors may interpret the same operations in different ways.
Incomplete research
When working to a publishing deadline there are always more sources that could be consulted or fieldwork to be done. There may also be space limitations imposed by the publisher which prevent full information being included. Consequentially, no work can really be considered complete. (Hence my view that the great advantage of web publishing is that work can be continually updated.) Also, new discoveries are always being made, both in the field and as new records come to light.
Propagation of myths
This has already be alluded to in that once a 'fact' finds its way into print (or these days on to the web) it tends to get repeated without question. The only cure for this is to go back to the primary sources. Even 'official' sources can be suspect - I've noticed factual errors in both the Sites and Monuments records of the regional archaeological trusts and in the RCAHMW site database. (In my own work I've tried to document the differences between my own research and the above sources.)
Primary sources
Historians are always told to go back to the primary sources. Unfortunately, for a lot of mining history the major source is the Mining Journal, which is hardly unbiased. Many of the reports originated from mine owners and operators in order to impress shareholders, particularly when more money was being sought from them - 'spin' is nothing new. Consequently, accounts of work done, tonnages raised etc. need to be treated with a degree of scepticism. More reliable are the Inspectors of Mines reports and Home Office Lists of Mines, although even these are not free from error. Probably the most reliable source I've come across were the actual field notebooks of one of the HM Inspectors of Mines - however, these only covered a fairly brief period in the early 20th century.
What can be done?
Trust no one! Check everything! There is not a great deal that can be done about past mistakes other than to try not to propagate them and to correct them in your own work. However, to make life easier for the next person who wants to work on the subject, please give full details of your sources, even if they're only conversations with the current landowner. This will at least allow others to evaluate the reliability of the information you present.
Sorry to go on at such length in answer to a simple question. If you have read this far, thank you for your patience.
Dave