To reproduce:
1. On the site home page, add a book.
2. View Book, then click on Import
3. Click on the Choose or upload a file... button
4. Error is displayed at the bottom:
Call to a member function get_id() on a non-object in /lib/blocklib.php on line951
The fix is built upon [#MDL-13276] Fatal error in files/index.php - Moodle Tracker, which is now implemented in Moodle 1.9 files/index.php but does not seem to have made it into the /mod/book/coursefiles.php which is based on files/index.php.
Anyway, find the line (probably 46) with:
if ($course->id == SITEID) {
Then add:
if ($course->id == SITEID and !$choose) {