When using an exp tome on any non instanced map, the experience gained when using an exp tome is considerably lower than expected. The rate at which it differs is considerably more pronounced on higher level maps. Because the item advertises 30% more experience from monsters, you’d expect to find the experience gained increased by roughly 30% while active, but in reality the value is considerably lower.
The reason I believe for this is because each map has a hidden bonus exp value, as high as +400% experience on some maps. On those maps, using the experience tome adds to the experience bonus of the map.
For example, a Red Deadborn Scap on the Storage Quarter map is shown to have 2712 experience in the adventurers journal. Storage Quarter has a hidden experience bonus of +311% exp. That means killing them on that map nets you 11,146 exp. Now if you toss on a token for +30%, you get 11,959 exp, and using a exp tome will boost that further to 12,773 exp. That means that the exp tome here is only giving you 6.8% bonus. This only affects field maps, as instanced dungeons and missions do not have an experience bonus.
Though this makes sense logically, these facts are hidden from the player, which in turn cannot make educated decisions about an item that costs real money. The item is advertised as 30% more experience, but in fact in most cases gives considerably less.