if you search Devs was very clear on what lvl tomes drops in loot. Loot bonus weekends and Loots Gems are WAI.
I did some digging but, its been a long while I'm guessing somewhere between Update 17-18 there was a clear statement by the Devs. "+4 tomes drop at X level chest with a chance to be +4-+5 Upgrades" Highroads during the loot bonus weekend with a loot gem going was just the right level. *found it http://ddowiki.com/page/Update_16 right in the release notes
Tome Upgrades can now be found in random treasure. These wondrous items will allow you to upgrade an existing inherent bonus in an ability score to the next higher inherent bonus. Upgrades that turn a +4 Inherent Bonus into a +5 Inherent Bonus will be able to be found in treasure levels of 27 and higher.
btw I did not "pay my way" with loot gems I used loot gems that I saves from the weekly store coupons, etc...