This source is not very useful to actually determine the success of D-day. However, it is very useful to say how exaggerated the leaders were, and what the general public believed at that time.
Source I was written by historian A.J.P. Taylor in 1975. A historians job is to ultimately uncover the truth about what happened in the past, Thus being a more balanced source of information. Also, the time at which this was written was about 30 years after D-Day had finished. Therefore this was a good time for an overview to take place. Taylor wouldn't really have an agenda for his writing, making it a more balanced view. However, it seems that Taylor didn't like Montgomery, making unfair, sarcastic comments against him "Things did not go altogether "according to plan" despite Montgomery's usual claim that they did." Taylor compared the landings to Gallipoli, which was very unfair because Gallipoli was a massive WW1 disaster, and D-day was defiantly never as bad as that. Additionally, Taylor exaggerated with the word dallied. The British did not dally, even if not everything went to plan.
Likewise, this sources is not very useful for telling how successful D-day actually was either. Nevertheless, it is useful to see the negative things about D-day.
In conclusion, I believe, if you put the two sources together, you would get a much more informative and altogether better source. Which would be better to answer the foremost question. Also, it really depends on what you want to know about D-day to say if these sources are useful or not. For example, source A is good for saying what the general public believed at that time, but it isn't very good at actually showing you the truth of what happened.
In preparation for the invasion, artillery equipment is loaded aboard LCTS at an English port. Brixham, England. 1 June 1944.