If you want a device for reading books, get an e-paper Kindle or Nook for sure. I have many tablets/e-readers (I develop mobile apps) and the two that get the most personal use are my iPad 3 and my Nook Simple Touch. Reading on the iPad is ok, but the e-paper screen on the Nook is amazing for reading text. You can also use Instapaper (and other Android apps) on the Nook which is a nice advantage over the Kindle.
I've got an ASUS Transformer + dock and I'm thinking I'm gonna trade it in for the Nexus 7 soon (or it's successor if I hold off for a while). I've also got a netbook that's way long in the tooth and I wanna replace it with a Win 8 convertible but I want something larger that's more comfortable to work with for a few hours, probably something 11.6-12" rather than 10".
I do find the TF cumbersome for web browsing after a while, I often opt for my phone when I'm on the couch instead of the tablet... And on many a plane ride I think I would've been happier w/a 7" that I can easily hold with one hand. So yeah, 10" is fine if it's your only secondary device (after phone & laptop/desktop) but if you're gonna splurge on several devices I do think 7" is a very attractive form factor.
I wish I could hold some of the Samsung/Amazon 8.9" tablets to get a feel for those but the former have disappeared from retail and the later are only sold direct obviously. The e-paper display devices are definitely worth looking at if book reading's your main usage case for a smaller device tho, since they cause less eye strain (no backlight).