my c program will only read the first line of my file

Your code is overly complicated and wrong.

You probably want this:

void readInBooks(Catalog *cat, char const *filename)
{
    FILE *fp = fopen(filename, "r");
    if (!fp)
    {
        fprintf(stderr, "Can't open file: %s\n", filename);
        exit( BAD_INPUT );
    }

    char *line;
    while ((line = readline(fp)) != NULL)
    {
       printf("Line: %s", line);
    }

    fclose(fp);
}

There maybe more problems elsewhere though.

There are also some problems in readline, I let you find out what as an exercise. Hint: it leaks memory.