First of all, when you are talking about pixel density of a device, you should be aware that most people know that the higher the pixel density, the better the clarity of the screen. However, what most people do not know is that this does not necessarily mean that it is better for you or for the device. It seems excessive pixel density could seriously affect the shelf life of the device's battery and thereby affect its overall performance. What's more, if your vision is not too good in the first place, then what do pixels matter?! What is more important is how sharp your vision is and the distance between your eyes and the screen.
Coming to the Asus Fonepad 8 which you mentioned, note that the IPS display is of 1280x800 pixels. Detailed analytical tests by experts have revealed that the Asus Fonepad 8 does provide sharp pictures, good brightness and vivid colours, though there does seem some problems with blue & red hues. Like any other similar e-reader, the Asus Fonepad 8 can create problems of clear reading in bright sunlight, but overall it does seem to be good for reading due to fairly good visibility at various angles. You can always check the device in a retail store first before making the actual purchase. See if the font size of text can be increased for better visibility of sentences and comfortable reading.
Also, if you are looking specifically for the Asus brand, you could check out too the Asus Transformer Infinity. Reviewers of this device have considered its 10 inch screen with 224 ppi as the ideal one for reading as it incorporates a brightness boosting feature to make e-reading better in bright light scenarios. You can install e-reading apps on it, including the one which comes with the device itself. Of course, it is possible that it is not within your budget, but check it out nevertheless.