One of the primary sources of material used by atheists for “Jesus = <insert god here>” is J. Frazer’s The Golden Bough, 1911. In my experience, whenever I look at this kind of material, it is always worth checking what Frazer said. In my case I want to find the origin of some curious statements about the Adonia.
His book is well referenced, if you go directly to it. He’s the only author I have ever come across to know of 6th century Syriac author Thomas of Edessa, or to have read the Latin version — the only version ever made — of his De nativitate. Yet read it he must have done, for he refers (correctly) to the fact that Thomas says the pagans even in his day celebrated a solar festival at Christmas-time.
A single volume abridgement is accessible at Gutenberg. But it came as rather a shock to discover that the original was actually in twelve volumes! I tried to locate this on Google books in vain; a search on Archive.org was better.
To locate all 12 volumes, use this link.