I second this. I prefer reading on pdf, so I bought the ebook[1]. The book explains indexing perfectly, and it shouldn't take you more than a day to finish. I can't recall a book having a better benefit/time ratio. I wish the author would release more books in this vein, but he hasn't. So right now I'm looking at Designing Data-Intensive Applications to learn more about different kinds of databases [2].
I had a Samsung Galaxy Note 12.2 for a while, but the built in reader (iBooks) in the iPad is far superior to everything I tried on Android over a period of months.
I used to have a high res inexpensive tablet that I loved to bits, but I dropped it carelessly and I'm finding it impossible to replace the screen. Right now I use Preview on macOS. Since I'm on my laptop anyway, I make notes in a text editor. That helps me remember what I read.
Kobo Aura One. Bit more expensive yet high quality.
Preview on macOS is also good.
EDIT: I don't know, when that review was written the device was just released. The device received various firmware updates since then. I convert everything to EPUB with Calibre.
I really wish there were at least one mass market device that wedded a good eink screen to a reasonable CPU / RAM. There was a crazy expensive Sony PDF reader. But I really just want a Kindle DX (Aura One okay too) sized screen that doesn't struggle to render PDFs.
[1] http://sql-performance-explained.com/
[2] http://dataintensive.net/