[chaley]

For #2: BetterRed's answer is probably sufficient. However, to do exactly what you asked for -- in the tag browser only show items in selected books -- do:

First, ensure that the option Preferences / Look & feel / Tag browser / Hide empty categories is checked.

Then:

1. If there are any marks, clear all marks. If you do this a lot then you might want to give clearing marks a shortcut.
2. Select the books.
3. Toggle marks on selected books (Ctrl + M)
4. Search for marked:true (Ctrl + Shift + M)
5. Press "Ctrl + *" (on my keyboard * is Shift + 8, so the sequence is Ctrl + Shift + 8) to create a temporary VL.

The tag browser will show items (authors, tags, etc) in the selected (marked) books.

You can make a shortcut for clearing the current virtual library. Preferences / Shortcuts / Miscellaneous / Clear the Virtual library.