Left side of assignment operator must be variable or property

[Bottlebank]

Hi guys, thanks for all the help so far, i'm hoping this is an easy one for you to help me with.

if (B_AuthorBox.text = "" && B_TitleBox.text =""){
        BookVar = BookVar;
    } else {
 BookVar = BookVar +1;
}

I keep getting this message

<b> Left side of assignment operator must be variable or property</b>

How can I get around it?

Thanks in advance

Bottlebank