Lasker's Manual of chess was the first book in the history of the game to succinctly describe what was actually going on with the pieces at the move level. Others talked about grand stratagems - but Lasker gives the beginner, the intermediate player and the master a brilliant insight into the importance of understanding the changes that go on wiuth different types of move - he shows that all chess positions are either favourable - when you must attack - unfavourable - when you must defend (and he means MUST) or level in which case you MUST try and increase the co-operation of your forces. Lasker argues this is a UNIVERSAL law of the universe and can be applied to any siutuation - not just chess. He argues successfully that a player given this insight and further advised on the method of attack, defence or increasing co-operation will be able to put up a good fight against a master.
The fundamentals of chess have not changed since Lasker's time - but many books make the mistake of giving a true theory of chess - this is what Lasker set out to fo here - by building on the work of Steinitz, adding his own propositions - such as The Priniciple of Proportion (meaning the attack, defensive action or increase in working together of you pieces must be in proporation to the advantage or disadvantage you possess) and giving numerous examples from play - the theory builds up allowing players to glimpse at the understanding the true master possesses.
Advances in chess technique have been made since this book was published, but there has been no advance in the basic buidling blocks of how chess actually works, and for this reason Lasker's Manual of Chess is an essential work in any chess enthusiasts library. Probably the most important chess book of all time, and do not belive me - ask the Russians, this book was standard reading in all Soviet Chess Clubs and little known or almost ignored by the West - and look what happened.