If you are playing an AL wizard, it is highly likely that the majority of your spells were added to your spellbook -completely free-.
You are correct that the six first-level spells you begin with as a wizard, plus the two spells you gain every time you advance a level in wizard, are free to put into your main spellbook. This does not mean those spells have no value.
On top of that, spells in your specialization school cost less to scribe. Retroactively calculating how much a wizard -would have had to spend- to make their spellbook is impossible to calculate in most cases.
Actually, it's far from impossible and usually very simple. One way to do so would be to simply calculate how much it would cost to make a backup copy of the spellbook, which is simply 10gp per level of spell in the book (half of that for spells of your specialist school gained after level 1). I mean, unless a wizard had the ability to identify whether a given spellbook was a 'primary' or 'backup' spellbook, there's no real justification to have the two types of spellbook have different values, even if they have different creation costs.
Another way to calculate the spellbook's value would be to simply treat all spells as if they had been scribed into the book manually (i.e.: not using the 'free' mechanic) for the purpose of determining the spellbook's value, which would be 50gp per spell level, half that for spells of your specialist school gained after level 1.
Edit: I'm actually OK with not bothering to figure the specialist bonus in the 'value' of the spells, even though they affect the 'cost' of the spells -- those are class features, and it's reasonable for a character to gain a benefit from a class feature, in this case, a more valuable spellbook than she paid to create.
There are numerous ways to calculate the value of a spellbook; that the rules aren't specific on how to do so in no way means that spellbooks have no value.
I was being generous saying filled spellbooks are only worth the value of a spellbook. They actually have no specific value at all, and can only even be kept by wizards because AL made a specific rule to allow them to.
As noted, this is a flawed analysis -- all spellbooks in AL modules are listed as treasure, and as noted in the ALPG, only certain kinds of treasure are "automatically converted to their full value in gold" at the end of the module (ALPG, p.6 - specifically, gems, art objects, jewelry, and trade goods). The 'rule' you describe is a rule that allows PCs to keep certain mundane items in those categories if they'd rather have the item than the gold (so you can keep the earring you got off the dragonborn sorcerer as a trophy, if you're willing to accept that in lieu of gold).
This is an exception to the usual rule that mundane items are either destroyed or liquidated automatically. A spellbook that is not claimed by a wizard creates precisely 0 gold to each party member.
Sorry, but this isn't the rule that covers spellbooks. You'll want to note the paragraph just below this in the current Season 3 ALPG:
"
Arms, armor, and other equipment fetch half their cost
if you sell them." (ALPG, p.6 - bold in original, italics mine)
And if you look on the list of adventuring gear (i.e.: other equipment), you'll find "spellbook" right there, plain as day. At the very least, a spellbook should be worth a minimum of 25gp as treasure, since that's half of what the equipment list specifies is the cost of an empty spellbook.
For example, the admins said it was legal to cast Fabricate to make a copy of a spellbook, but there is no value for them.
I'm unable to find any comment on the value of a spellbook created by Fabricate -- can you provide a citation? Also, note that the spell description of Fabricate does require raw materials to exist to Fabricate into an object; it would not be unreasonable to require that a spellcaster who wants to Fabricate a spellbook provide the same exotic inks that would be required to copy the spellbook manually (i.e.: 10gp per spell level of existing spells in the book). Otherwise, you'd only be creating a blank spellbook out of random paper and binding materials.
Everyone's mileage may vary, but through 40 sessions in season 1, including 3 Expeditions, my wizard never encountered a single wizard scroll or spellbook until he got Azbara Jos' spellbook from chapter 6.
Oh, well that makes sense, then. You're playing through a hard-cover adventure, with only a minimal amount of actual AL-crafted material. It's hard for me to imagine what kind of solution the admins could come up with to correct this perceived issue, given that the admins have no say in what appears in the hard-cover adventures and no real authority to change them.
Play through 40 sessions of actual AL-produced adventures, and you'll see far more spells for your spellbook. Heck, there are spellbooks in five consecutive Season 1 Expeditions, including one with multiple spellbooks.
With that said, it would be helpful if future modules could include the gold piece value of a spellbook on the list of treasure at the end of the module. This would allow the DM to easily convert the spellbook to gold should no one in the party want to claim it, rather than allowing it to 'disappear' as valueless. This would avoid the problem where the DM would need to assign a treasure value without much guidance on what value makes the most sense.
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Pauper