I recently discovered that Stat Tomes go up to Tier 20. The last time I looked at them it was Tier 18.
Update 25.4 introduced Tier 19 to 20 Stat Tomes, levels 15 to 20 are all available from within a Remmorchant Stat Tome Box.
All Stat Tomes 1 - 20 (Ranks 1 to 20) are also available from a Stat Tome Box for 525 LOTRO Points.
Hmm, another reason to do Remmorchant.