I'm afraid we have a number of cynics in our ranks on this board
Actually, having been a full-time glass dealer specialising in British Glass for the last well, nearly 26 years, I think that there are indeed areas that you could collect and get a good return, providing that you are looking at a mid to long term investment. A minimum of five years (in most cases) will mean that you get your original investment back. This is a rule of thumb and naturally can vary, according to the item. I have known some items that have gone up throughout the twenty five or so years, and others that have dropped off the scale.
My suggestion would be that you use a good quality dealer, who has a track record, who can advise you and give you the confidence to back you own judgement over a period of time.
Like many investments, glass has its ups and downs and fashion can play its part, so the use of a dealer can pay you back through their knowledge of the subject. However, if your aim is to enjoy yourself while collecting then you need to do some research and look for areas that interest you and give you pleasure.
The scatter gun approach can work, as can the bootfair/charity shop circuit, however both have their shortcomings, which usually entails gathering large numbers of items together, but with only a small proportion having lasting
value. This is not to put down that way of collecting, just to give advice, Some folks have been known to have a lot of luck through both methods, but that is not always the case if you are thinking in terms of an investment.
Be judicious and buy quality items in good condition! And, develope a good 'eye'.
Lastly, but importantly, only have this as part of a diverse portfolio of investments. It never does to put all your eggs in one basket
Kind wishes, Nigel