@Hihahiho
Pre-pubs are something like drafts. They already have some quick QC but may contain some spelling or grammar errors.
Pre-orders means you pre-order the published E-Book. The published E-Book is more complete then the pre-pubs. It will have the color inserts and the errors will be mostly gone. I say 'mostly' since although rare some mistakes do happen.
There are 2 ways to buy E-Books. Either directly from JNC or other retailers. Other store you get a normal E-Book. But directly buying from JNC will get you a Premium E-Book. A Premium E-Book is DRM-Free which means that you don't need a specific app to read it. It also has the highest resolution for the images compared to anywhere else. It also contains extras like bonus short stories, text-less illustrations, bonus illustration, and some other stuffs that can't be found anywhere. FYI, only a member can buy from JNC.
Regular Membership = $7 per book
Premium Membership = $6 per book
Other Retailers = $6.99 per book
Lastly, pre-pubs expire and are not permanent. The only available pre-pubs are volumes that has not been published yet or currently being translated. In every start of the month a "Catch-Up" happens wherein a select few series have their pre-pubs unlocked and it will be possible to read all the volumes for those series for that entire month as long as you are a member.
As for membership details:
Normal - you can read every pre-pubs that are available and can buy directly from JNC
Premium - same as above but when buying E-Books from JNC you get $1 discount and you also get a Premium Credit every month on the 15th. This credit can be used to buy any books directly from JNC.
Normal Membership = $4.95 per month
Premium Membership = $10.95 per month (This is basically $4.95 for the membership fee plus $6 for the monthly credit. So your actually just paying the membership fee while additionally buying a book every month.)
Advice:
If you just want to read then a Normal Membership is enough but if you want to own the E-Book and read it in one sitting and not every week then Premium is better since buying books in bulk is cheaper with the Premium Membership.
Premium Membership vs Other Retailers Cost Breakdown:
PM
$4.95 = membership fee
$6 = monthly credit
$6 * 4 = $24 = additional credits
Total = $34.95 = 5 books
OR
$6.99 * 5 = $34.95 = 5 books
As you can see if you buy more than 5 books per month or more than 60 books per year being a Premium Member is cheaper. Plus you get freebies that can't be found anywhere.
