Thank you for your inquiry. You are absolutely correct that the Old Testament geneology lists Jacob as the grandson of Abraham (and son of Isaac and Rebekah). You are also correct that on page 56 of "Heroes of History" Jacob is referred to as "Abraham's oldest son." This was an error that Dr. Durant did not live to amend and was missed during my pass through edit of the scripts that became the book "Heroes of History" and, indeed, by the editors at Simon and Schuster.
As Dr. Durant was very familiar with both Old and New Testament history, there is little doubt that he would have caught the error himself (indeed, he did correct it during his read of this particular chapter on tape), had he sufficient time to return to the project. As you are probably aware, Dr. Durant had initially intended the "Heroes of History" project as a series of what he termed "mini-talks" that were to be recorded onto audio tape. He and his wife and collaborator, Ariel, had recorded 14 of the talks when Dr. Durant had decided to turn the project into a book. Each book that Will Durant wrote took approximately five years to complete; he would research and take notes for two years, write and rewrite for another two, and then refine the manuscript during the final year. During this time he would also submit the manuscript to certain scholars whom he held in high regard so that they might point out omissions or make other constructive criticisms of the content.
Sadly, the manuscript for "Heroes of History" never made it to that point and eventually ended up in the hands of yours truly, who, while appreciative of Dr. Durant's art, erudition and labor, was nowhere near as knowledgeable on matters such as historical geneologies (among other things) as the good Dr. was.
There were other typos that were caught (and some that were not) prior to the book's first printing and (I would subsequently learn upon listening to the recordings) there was even some additional material that was not included in the original scripts (later, the "chapters" of "Heroes").
I caught the error you pointed out shortly after I received my author copies of the title and had opportunity to re-read the book after it had been published. I passed the need for amendment along to Simon and Schuster and they have informed me that they will correct it in the next printing.
As I mentioned earlier, it is highly unlikely that the oversight would have endured had Dr. Durant lived long enough to return to the project. It's inclusion in the book is as a result of my oversight alone and I appreciate very deeply your keen eye and query about this matter.