I want to produce ebook covers and printable covers. So, which apps can I use to draw, without any doubts, in 300dpi or higher using my tablet?
You don't need 300dpi images for print. This is a myth perpetuated by those who don't understand digital image resolution. For more detailed information read The Myth of DPI.
What you really need is an image of sufficient pixel dimensions so that when printed at 300dpi, at a specific physical size, it will be of sufficient quality. You don't need a special app to achieve this. Almost any digital painting/drawing software that allows you to set up a custom size document can be used.
Let's say you have a book cover which is 11" x 7", and this is the size of print you need.
Multiply each dimension by 300dpi. This will give you the number of pixels for each dimension. In the example case, that will be 3300px x 2100px. Then set up a new document in your favourite painting/drawing app, with those pixel dimensions.
The drawing/painting you make will now be of sufficient quality for printing at 300dpi. The actual DPI setting itself is irrelevant. You'll understand why after reading the linked article above. The DPI setting can be easily changed in almost any decent image editing software.