That works great until they take the Expanded Spellbook feat and get to choose 3 spells. 8)
From my post on the other wizard spellbook thread (About wizard/warlocks):
From the reading of the Spellbook text, and the section on powers(pg 28/29), it only mentions removing spells from your spellbook when you replace them via retraining or leveling at 15, 19, etc. But it says 'one spell' even in the spellbook section. So when you hit 15, you replace 1 spell in your spellbook(maybe 1st level, but it doesn't matter) with the 15th level spell. You leave other spells of that level alone. To get additional spells of 15th level, you need to 'retrain' to swap them out, at 1 per level. So at 15, you could have 2 15th level spells, if you used retraining to swap in the second one. At 16, you could have 3.
Hrm. From reading in detail; you get to choose new daily attack spells at 1, 5, and 9 only. 15th, 19th, 25th, and 29th are all basically 'free' retrains. Thus you'll only ever have 6 daily spells in your spellbook, unless you take the feat Expanded Spellbook, which lets you have 9 Daily spells. Similarly for Utility powers, you'll have 10 (expanded spellbook doesn't do anything for Utilities); the powers you get from your paragon path or Epic Destiny do not count as 'levels where you get to select a spell' since you are given exactly one spell to take; there is not a choice.