Books/resources on Scotch (as white) and Petroff (both sides but mainly as black) Could someone recommend books or online resources on those two openings for intermediate players (about 1800)? 

Since my opponents at this level are likely to go out of theory on move 4 or 5, I want to have the knowledge to see why their move isn't that good and how to respond to it. 

For example, 1 e4 e5 2 Nf3 Nc6 3 d4 exd4 4 Nxd4 d6. Here I used to trade 5 Nxc6 bxc6, and double black's pawns, but I'm told it actually helps black to open the b file and have an active middle game.

So I would like resources that have more explanations on the opening's idea/planning, not just comprehensive list of variations. 



Edit: I bought the book "The Petroff: move by move". After reading for about 30min, I put it down and disliked it. The reasons are


This book is supposed to be an opening book. But it runs all the example games to the end. The author should make a judgement when opening is finished, provide evaluation of the board and plan for middle game, then go on to next example.
On one page, the author writes (for the Cochrane gambit) "(white's) Bc4+ should always be met with d5 since after Be6 Bxe6 Kxe6 black's king is pulled further to the center". And on the next page when white plays Bc4 a move later, black plays Be6, then the game continues Bxe6 Kxe6. No explanation here!
There isn't any dedicated section for game plan/pawn structure/piece placement etc. These advice are scattered in the example games. Moreover, its explanations are more towards "why this GM's move is good", not "guidelines to find good moves".
I can only make recommendations for the Petroff Defence, as I have never played the Scotch:


The Petroff: Move by Move by Cyrus Lakdawala. An engaging author using a question and answer format in annotated games
Petroff Defence by Alexander Raetsky and Maxim Chetverik. This book is more orientated towards Black's point of view, but the explanations are accessible to the club player
thechesswebsite YouTube video on the Petroff Defence