function foo() {

    return createGroupMembers(parsedChat).then(function(val) {

        var members = val;



        return createMessages(parsedChat, maxPages).then(function(val) {

            var messages = val;



            return Promise.all([createFrontCover(subject, firstdateOfMessages, lastDateOfMessages, isPreview), createStats(parsedChat), createBackCover(parsedChat)])

                .then(function(results) {

                    var front = results[0];

                    var stats = results[1];

                    var backcover = results[2];



                    var book = head + front + stats + members + messages + backcover;



                    console.log('pages in this book: ', pages);

                    console.log(book); // logs perfect values.



                    return book;

                });



        });



    });

}

Now foo will return a promise which can resolve to the value of book

foo().then(function(book) {

    console.log('huzzah I have book ' + book);

});

To be honest, foo could be rewritten, but that's a different question altogether

FYI: you could do something like this for foo

function foo() {

    return createGroupMembers(parsedChat)

    .then(function(members) { // members

        return Promise.all([members, createMessages(parsedChat, maxPages)]);

    })

    .then(function(members_messages) {  // membersMessages

        return Promise.all([createFrontCover(subject, firstdateOfMessages, lastDateOfMessages, isPreview), createStats(parsedChat)].concat(members_messages, [createBackCover(parsedChat)]));

    })

    .then(function(results) { // front, stats, members, messages, back

        var book = head + results.join('');



        console.log('pages in this book: ', pages);

        console.log(book); // logs perfect values.



        return book;

    });

}

Messed around with the order in the second (was your only) Promise.all, and added the previous Promise results in it to make the final conatenation of parts as simple as a .join - doing it this way will also propagate any erros correctly, so your usage of foo can be

foo().then(function(book) {

    console.log('huzzah I have book ' + book);

}).catch(function(err) {

    // handle any and all errors here

});