If you want to find study-guides, cross-references, etc. for a given book, a starting point you might consider is http://bahai9.com/Writings
In this case, the above should lead you to http://www.bahai9.com/wiki/Promised_Day_is_Come
Unfortunately, in this case, we don't have a formal cross-reference to sources. However, you might find the chart at http://bahai-library.com/?file=zamir_sl ... .html#slh3
helpful in going backwards from Summons of the Lord of Hosts to Promised Day Is Come (though not for the first pages of PDC as they are not in the chart) since PDC quotes from the Tablets in Summons quite a bit...
If you are familiar with wiki pages (or want to get familiar), you can also edit the PDIC page at http://www.bahai9.com/wiki/Promised_Day_is_Come
yourself to add any cross-references you find for the next group who comes along...
I added a few as a beginning to the "Original sources by paragraph" section there... (You have to register to edit, and page versions are tracked and can be reverted easily, etc. as at Wikipedia)