Obviously with engines being heavily used in CC you can’t play the same lines you might use OTB because some are simply too risky and engines will see right through any ticks and traps. Most high level openings being used these days are super solid and endgames are not unusual. Be prepared to play long games! On most CC sites players are free to consult chess publications or literature in printed or electronic form.
Stan Vaughn, a well-known correspondence chess player offers the advice that you should NOT take opening variations in opening manuals for granted. He recommends going to the end of the line, set up the position, then check it out on a computer first and try to choose lines that give you the best positions when reaching the middlegame.
He recommends finding lines that are deceptive, which if you survive will give you, for example, a queenside pawn majority. Unfortunately such judgment is probaly beyond the ability of most of us. Vaughn points out engines do not see that in an endgame 50 moves down the line a pawn majority may give a winning advantage.
Also, remember game databases are like museums in that they tell you what was popular in the past and many times improvements will have been found. This means a LOT of research, engine analysis and constantly updated opening books and databases are a must. Speaking from experience, a while back against a 25oo-rated CC player I entered a line in which the two year old game played between two highly rated CC players in my DB showed me having a significant advantage. After some 20 moves, in my analysis I found an improvement for Black that turned the tables and gave my opponent the advantage. Of course he found it, too, and I was left struggling for many moves and eventually lost the ending. Better research on my part would have avoided such a situation.
The 7-part ChessBase Workshop series by Steve Lopez on correspondence chess gives a lot of information on correspondence chess and practical examples of opening analysis. Lopez shows you how to interpret the information Chessbase is giving you. If you don’t have Chessbase you can download the free version from the site. Or, you can apply the principles to the opening book with whatever software you are using.
One important point the author stresses is ChessBase isn't going to make decisions for you; it's not going to tell you what to play. That's not its purpose, and never was. ChessBase is simply an information storage and retrieval system.