I noticed some of the 3.5 manuals refer to books like Manual of the Planes and Oriental Adventures, but when one of my friends found those books, they were 3.0 edition. Is that right, or were there 3.5 reprints of books like these?
There's a rule for this!
Some 3.0 content is still 'current' for 3.5. The official rule from Wizards of the Coast is that any content that didn't get updated (that is, printed into a new 3.5 book) is still valid and is thus usable with 3.5. Oriental Adventures and Manual of the Planes are indeed two such books, requiring only minor adjustments to the skill system(s) present to be in accordance with 3.5's slightly smaller skill list.