Books for beginners of (correspondence) chess:

About the books I would like to recommend Reuben Fine's "Ideas about Chess Openings", as he explains why certain moves are played in chess openings. It might be a little advanced for you, but given that you want to understand why you have to play certain moves --- rather than
memorizing them --- it might be a good choice.
The book is dated, but since you are not playing on the highest level, that does not matter.


You might also want to buy one "general opening reference". These books just give "lines" of openings to play without much explanation. It gives so to say a standard with an evaluation of the outcome.

Nunn's chess openings is a another book on chess openings that got good reviews. I haven’t used it myself, but it might be the better choice.


I myself have a copy of Modern Chess Openings, which helped me in the beginning to choose lines (but also to investigate what others might play). The newer version (14, I have 13) can be found under the link above.


Another book you might like is "Logical Chess: Move by Move" by Irving Chernev. Here chess games are fully explained.


In a similar fashion John Nunn has written a book, his games are more recent (1990s). I have read good reviews about the book.