This is a post that will be incorporated in the updated Familiars' Handbook.
Imbue Familiar with Spell Ability
Imbue Familiar with Spell Ability is a spell that's very much alike Imbue with Spell Ability, but instead of affecting any target, it's specialized to just target a familiar. The spell is located in the sourcebook Complete Arcane and unfortunately it wasn't included in Spell Compendium, if that means anything to you. The biggest benefit you're getting by using this spell compared to the previous two (Spell-Linked Familiar and Imbue with Spell Ability) is that your highest spell level that you can transfer to your familiar is greatly enhanced: the cap with those aforementioned methods was 2nd level spells and with Imbue Familiar with Spell Ability it increases to 5th. The spell itself is a 6th level spell, which means that normally you're going to get it around level 11, assuming that you're not getting behind on your spellcasting from multiclassing; now you are able to imbue your familiar with one spell per three caster levels with a maximum spell level one-third your caster level (maximum 5). The first thing someone notices is that while the maximum spell level has a cap, the amount of spells you can transfer does not have one. In addition the spell mentions a caster level, which probably refers to the spell's caster level, which can be modified by certain feats and effects, say, using the feat Arcane Thesis from Player's Handbook II; this way you can use some metamagic tricks to lower the spell's effective level (for example, using the infamous Sanctum Spell from Complete Arcane), to lower the spell slot you're investing and increase the potence of the spell's effect.
Further reading Imbue Familiar with Spell Ability we find that the spell's caster level functions as if you have casted it, at least when determining the variable characteristics (which rules out spell difficulty class at least), but still, that's a very good tool in order to do the job that this series of articles explores: giving your familiar access to your spells in order to benefit from the extra actions and versatility, finally! This is the feature you'll want to use (maybe in addition with the previous two methods) in order to transform your companion to a spellcaster: there are virtually no downsides to using it, besides of course of the spell slot you're investing, as your caster level remains the same, there's no nonsense restrictions, like xp costs, material components or required foci.
The only weird interaction that is worthy of mention, is that: "The transferred spell's variable characteristics (range, duration, area, and so on) function according to your level.". So, the question here is: do the benefits from special abilities and feats gained from levels work on your spells? If the answer is 'yes', you can argue that almost everything is derived from levels (well, ok, everything besides racial characteristics and stuff that others do to you): for instance casting a spell to gain a certain benefit, IS an ability that's gained from levels. The answer to the above question is probably a 'no' unfortunately, at least, that's what I would rule; however, even in that option, there are certain benefits; for example, take a character that has levels in the unseen seer, so his caster level for some spells is less than his actual level - but his transferred spells work equal to his caster level, at full power! The same is true for multiclass characters, or for characters that want to transfer spells from other classes - there is no restriction that the transferred spells must be from the class you have casted this spell from (most likely sorcerer/wizard), not even that it must be from an arcane casting class; a wizard/ur-priest/mystic theurge build can thus benefit from having a familiar and also enjoy ur-priest spells that function according to his level, not his special caster level that's calculated according to the prestige class's entry.