1. In addition to the performance component (31.5-38.5 minutes), there is the Viva Voce: you need to have programme notes (990-1210 words) prepared and be able to discuss your pieces, the composers, musical context, form, etc. with the examiners. Then there is the Quick Study: you have 5 minutes to learn a piece of about grade 6 difficulty and two pages in length. I don't know of any actual books, but to prepare I just looked at the grade 6 syllabus and tried pieces from there.
2. It depends on where you are from; in my part of the US, it costs ~$250...
3. I think this is subjective. My teacher told me to treat it as though I were giving a regular solo recital, so I wore a dress and nice flats (would have worn heels, but I'm paranoid about pedalling in them!). Then again, I'm comfortable in dresses - if I hadn't been, maybe I would have relaxed this somewhat. Basically, I think one should dress as well as possible without becoming uncomfortable.
4 and 5. These are difficult questions to answer because there is a huge variety in people who pass grade 8, and the skills tested in the diploma are a little different. I found grade 8 manageable - learned the pieces in 3-4 months (while also working on other things) and did decently well on them (26, 27, 27). The other strong component of my grade 8 exam was the last aural test (identifying musical period and discussing aspects of style) - the examiner specifically complimented this test and I think it was this and the pieces which led him to give me exactly 130 marks, even though the other aural tests and scales were weak. About two months later, my teacher proposed that I enter for DipABRSM the following session, on the grounds that my strengths for grade 8 were exactly the skills transferable to the diploma. This gave me 5 months of preparation time, which wasn't enough for me - I ended up failing the recital section by two marks. I retook the exam a few weeks ago (making it one year since grade 8), and though I don't know what the outcome will be I feel a lot more secure about it.
Sorry about the long-winded ramble, but the short answer is that it depends. The level of musical maturity expected for DipABRSM is higher than grade 8, but this aspect was not a problem for me since the examiners of both exams complimented my musical intentions. What did become a problem was the demands on technique, stamina, and ability to perform under pressure - though I was affected by pressure to the same degree in grade 8 and in DipABRSM, the difference in technical expectations and the sheer amount of music required meant that this pulled me below the passing threshold in DipABRSM. You need to assess your own strengths and weaknesses to determine how much of a reach DipABRSM is for you.
6. Not in the slightest!

7. Again, it depends. I personally feel that my playing has improved drastically - 1.5 years ago I never would have dreamed I was capable of performing a full, albeit short, recital programme at the level that I have. Performing for the diploma and in preparation for the diploma has gradually made me more comfortable with performance anxiety - though I struggle a bit, I can feel that I have improved, and my recent diploma performance was one of the least nervous I have ever had, with no memory slips!

(Memory problems have plagued me for the longest time...) I have more confidence in my skills, and much more music seems approachable to me.
This is what the diploma has done for me, and I don't even have the certificate. The certificate itself is of somewhat less use to me, since I'm not formally studying music, but I still think it might be useful if I ever want to do a bit of beginner teaching on the side. But for me, the main importance of the diploma was to serve as a goal. Others for whom music is a primary occupation can tell you about the "practical" worth of the diploma.