Which Xenon Special Mastery Book to choose?

I've already chosen Xenon Expert Special Mastery Book from the level 70 Xenon event.

What is the next best Special Mastery Book to choose aside from that?

Also, does Unleashed Coin Shop give Special Mastery Book or just gives a random one?

I got Beam Dancing since that seems to be the most expensive at least in my world.

Go check the prices of Mecha Purge, Beam Dance, OOParts, and Offensive Matrix in your world and get the special that is most expensive.

Unleashed Coin Shop gives only level 30 books, so it's not really worth it since there seem to be a lot of 30s on the market anyways.

I got Beam Dancing since that seems to be the most expensive at least in my world.

Go check the prices of Mecha Purge, Beam Dance, OOParts, and Offensive Matrix in your world and get the special that is most expensive.

Unleashed Coin Shop gives only level 30 books, so it's not really worth it since there seem to be a lot of 30s on the market anyways.

I'm terrible with prices but for big worlds like Windia what would be the most expensive one out of these choices?

Look at usefulness over prices. What do you need most? Prices and needs overlap, but you need to decide which is more important.

Bosser? Get Mecha Purge. Grinder? Get Beam Dancing. Offensive Matrix is always the next one to get after these 2 (that is, if you decide to dump one for the other).

Hadriel

I'm terrible with prices but for big worlds like Windia what would be the most expensive one out of these choices?

Well like I said, BD is the most expensive here. Other than that, listen to what Hadriel said (I personally recommend OOParts over Offensive Matrix though) and get what is most important to you.

Beam Dancing is the main training skill so I went with that. Realistically, one doesn't really boss until 180+.

OOPArts depends on how fast you recharge your supply, and how well you can juggle Hologram. You DO know that you can open one hologram immediately after another has expired and on cooldown? Very useful at low levels - but opening one hologram when another is out will remove that existing one. Tests your ability to switch modes within one Mecha Purge - Snipe delay.

Hadriel

I went with xenon expert, since at max it gives 30 WA anyway. Next is Offensive Matrix, followed by Instant Shrink and/or Ooparts.

OOPArts depends on how fast you recharge your supply, and how well you can juggle Hologram. You DO know that you can open one hologram immediately after another has expired and on cooldown? Very useful at low levels - but opening one hologram when another is out will remove that existing one. Tests your ability to switch modes within one Mecha Purge - Snipe delay.

Hadriel

Does server lag make it so that sometimes the cooldown is longer than expected? I remember that has been like that for some cooldown buffs, like Final Cut.

Also, for some reason when I use the Red hologram buff seems to like have the hologram disappear but the buff icon is still on top.

Does server lag make it so that sometimes the cooldown is longer than expected? I remember that has been like that for some cooldown buffs, like Final Cut.

Also, for some reason when I use the Red hologram buff seems to like have the hologram disappear but the buff icon is still on top.

What server lag does is to cause the ball to appear later than it should, and disappear as expected. The hologram seems to persist longer, but that's just visual because the ball will fly out lol. The Support hologram produces a buff that gives you that 15% extra HP, but the supportive effect of you gaining one supply per (ally and self) dodge will last only 10 seconds.

As for cooldown... sorry can't help you because you got to suck it up. But you have 2 offensive holograms to play with, so that's not a big issue for me tbh. Once I see the ball fly out, I'll go [Snipe --> Mode select button --> Right arrow --> Right arrow --> Up/left arrow arrow --> Snipe]. All in one delay. For reference, all bossing skills in 4th job are on the Up-position of the Mode selection. Buff expiration and cooldown expiration occur in the same tick, in some instances aligning with mob spawn ticks but I won't put all my money on that last one.

Hadriel