A quality e-reader is an extremely invaluable addition for a deployed individual. The ability to carry around a small tablet that can hold literally thousands of books is beyond incredible. We'll cover a few different options below.
If you're looking purely at cost, you can't go wrong with Amazon's Kindle Paperwhite E-reader. The Paperwhite is a great choice for many reasons. Its small and compact size means it can be comfortable carried pretty much everywhere. It has a 6 inch screen and measures at 6.3" x 4.5" x 0.36" (160 x 115 x 9.1 mm) meaning it can be thrown in the cargo pocket of one's uniform. Most contractors also wear pants with cargo pockets and the Paperwhite easily fits there too. At this size it can be taken into any situation with minimal impact to comfort or fit. You will barely notice it's in your pocket or back pack since the little guy is very lightweight coming in at 5.7 oz (161 g).
Battery life is phenomenal. If you venture into the fine print of the Paperwhite you will find that Amazon advertises a battery life of 8 weeks at 30 minutes a day. If my math is correct that means the device has a battery life of approximately 28 hours. Even reading 2 hours a day will give you about two weeks of reading on one charge. To get back up to full power you merely have to plug it in for 4 hours. Plug it in before going to bed and when you wake up, you have enough juice to keep you reading for another 2 weeks or so.
The Paperwhite has built in storage of 4 GBs. With ebooks having an average size of 1-3MB, you have probably more room for books than you could feasibly read over your deployment. If you want to do the math, 1 GB equals 1024 MBs. Bottom line, a whole lotta books. Another great feature of the Paperwhite is the backlighting. The backlighting is adjustable so you can set it to comfortable level without being forced to read with too much light coming back at your face. Another nice feature is WiFi. The Paperwhite comes with WiFi as a standard option giving you untethered access to download and shop for books without having to transfer from your main computer. Until recently the Paperwhite was only available in black, however Amazon has decided to offer up a white model as well.
Down side of the Paperwhite will have to be that it doesn't come with a USB cable. Realistically in this day and age, you probably have several charging cables and will be able to make due just fine.
The NOOK GlowLight 3 eReader seems to be one of the closest peers to the Paperwhite. This device is similar in size and weight to the Paperwhite, has twice the storage capability but just doesn't have as smooth of an interface as the Paperwhite. It also lacks the services that come with Amazon. I think Amazon just overall has the upper hand here and Barnes and Noble is woefully outmatched. I intended on doing a more in depth review on the NOOK but I just couldn't bring myself to do so. At $119.99 compared to the Paperwhite's price of 99.99, there's simply no reason to go with this more expensive option. Sorry B&N.
So by now we're all pretty familiar with iPads. This is a significant step up in price but we get much more than just a device for reading books. I specifically chose the Mini version because it is smaller and lighter than it's bigger brothers and that's great for portability. The small form factor can still easily fit in a cargo pocket and be taken anywhere the smaller eReaders can. In addition to allowing you to read your books on the go, you will also be able to load up movies, TV series, music, or any other media that your heart and mind desires. This specific model has 128GB of built in storage. That's more than ample storage for weeks worth of entertainment. This model also has built in WiFi, so if you find yourself in a deployed area where there are abundant hotspots, you can easily access your social media accounts to stay in touch with family and friends. You can download many of the same apps your phone contains, check email, browse the internet, the possibilities are almost endless. The biggest limiting factor if you choose to go this route will undoubtedly be battery life. I carry a powerbank most of the time to top off my devices when they're low so I can stay connected.