William's Lyne book Occult ether physics - Tesla's hidden space propulsion system and the conspiracy to conceal it - 2nd edn (1997) makes an attempt to deliver you Tesla's lost theories about ether and gravity with a special emphasis on how they apply to the electrogravitic propulsion systems which are used in flying saucers. Author gives you an overview on how ether theories evolved on the end of 19th century with an emphasis on Tesla's philosophy and his Dynamic theory of gravity which seems to still be a military secret inside USA. Here gravity is caused purely electromagnetically meaning that new propulsion systems can be made according to it (and have been). It seems that making such an engine was one of Tesla's most original ideas but powerful elites have prevented us from knowing anything about it. Instead we have other theories about space and time such as relativity which haven't produced anything useful so far.

From back cover:
Is there a "secret physics" with a different set of rules - hidden away from us earlier in this century, by a powerful elite who fear the technology based on it will strip away their power and wealth, and liberate us from their grip? Dynamite comes in small packages, and this little stick of scientific dynamite, blows apart pseudo-scientific myths and instills the truth.

Lyne - Occult ether physics - Tesla's hidden space propulsion system - 2nd edn (1997).pdf 9 clicks

Please share this book with others, add links on other forums as I don't know many of them.