What you can do, if you really want that +5 bonus as fast as possible, is this:Halivar said:
When hitting 17th level + 5,100 XP, make a Tome of Whatever +1. Save it.
When you again hit 5,100 XP over 17th level, upgrade the book to +2. Repeat until it's a +5 book.
OK, you're delaying your acquisition of levels 18+, but this at least allows you to skirt the rule on not spending XP that would make you drop a level. A slightly less economic version would be to make Scrolls of wish (or Miracle) instead (at a cost of 1,912.5 gp and 5,153 XP each instead of 1,250 gp and 5,100 XP for each bonus point in the book), but in a pinch you could use the scrolls to wish for something else.
