Per their site:
Diesel eBooks will be closing at the end of this month. It's been a great ride! We're exploring our options - eBooks are still in the infant years and there are many opportunities opening up now and in the future. We want to thank you for being such loyal customers. Understand this doesn't necessarily mean you won't see Diesel in another form in the near future.
IMPORTANT: you must download your eBooks here by month end. Downloads will not be possible on April 1st.
The Team at Diesel eBooks
From Books On The Knob regarding Diesel Ebooks & Eros closing.
"Some people apparently received an email about this, but I didn’t and just caught the news in a books forum: Diesel Ebooks (and presumably Eros, the sister company) is closing down at the end of this month. The notice is posted on their Support page and they warn that you need to download all of your books from there no later than the end of this month. The store has been having problems for quite a while and this isn’t entirely unexpected, but I would have expected an email out to all of their customers AND a notice on the front page, especially now that there is only a week to get any of your purchases backed out.
They are also apparently having problems with their content server and some books are already pulled from their shelves by the publisher, so don’t be surprised if you can’t re-download every one of your books. I just checked mine and could not get to all of my books (but did have all of them downloaded, which I confirmed while browsing my library there).
Because of this problem, though, I would not buy any further books there, even though they are technically open until the end of the month."