If you want a book on K98k sniper rifles, then you might consider buying all three volumes from the forum owners - they do contain quite a lot of information on the K98k sniper rifles too.
In case you are satisfied with a picture book then you might consider buying Robert Spielauers book which is still available as eBook on Amazon. It covers more than just K98k sniper rifles.
Generally it would be helpful if you can clarify a bit more if by "book on optics" you mean a book on scopes, how they work, which models there were, or if you mean on sniper rifles, or any specific nations or periods, ...