There are several grimoires that are exclusive to Modern Neopets and have the ability to alter your gameplay experience including giving you more inventory slots, more pet slots, changing the name and personality of your pets, and granting you treasure map pieces. These grimoires are arguably one of the biggest differences between Moderneo and retail Neopets.
Here's a quick rundown on what each of the grimoires do.
Allows you to change one pet's name, gender, and personality traits or swap two pets' names.
Hidden Tower price: 50,000 NP
Grants you 25 inventory slots, gallery slots, or shop slots.
Hidden Tower price: 100,000 NP
Grants you three random treasure map pieces (except Forgotten Shore map).
Hidden Tower price: 150,000 NP
Grants you one additional pet slot.
Hidden Tower price: 200,000 NP
Grants you one free dip in the Rainbow Fountain.
Hidden Tower price: 1,500,000 NP
All of the grimoires can stock in the Hidden Tower and be received from Random Events. They also sell at regular NPC shops but only stock extremely rarely, with the exception of the Mutare Naturam which stocks somewhat commonly.
Mutare Naturam and Spatium Procuratio are typically much, much cheaper in users' shops than in the Hidden Tower.
Meanwhile, the Occultus Loco and Animalis are usually cheaper in the Hidden Tower than in users' shops since these two are always in high demand.
Lastly, the Eritique Arcus is currently much cheaper in users' shops than the Hidden Tower. This is probably due to users received a free Arcus during December as well as there not being many new expensive pet colors released recently. The price of the Eritique Arcus will most likely increase later on in the year.
Always check the price of the grimoire before buying, even if it looks like a good deal.
A note on other grimoires
Although they also stock in the Hidden Tower, the Grimoire of the First Order, Grimoire of Prosperity, and Grimoire of Affluence have nothing to do with the other Moderneo-exclusive grimoires mentioned in this guide. They don't change anything on your account if you use them. They exist just to be read to your pets like normal books and to get avatars.
The magical energy swirling around this tome seems to change the personalities and attributes of those around it.
Using this grimoire allows you to change a pet's name, gender, and personality traits.
The Mutare Naturam occasionally stocks in the following NPC shops for 10,000 NP.
It can also stock in the Hidden Tower for 50,000 NP.
As you can see, there's a huge price jump. Never buy the Mutare Naturam from the Hidden Tower. You can almost always find it from users' shops for around 10K NP.
The Mutare Naturam can also be found infrequently through Random Events.
Notes on usage
You can also use the Mutare Naturam to swap two of your pets' names. To do this, simply use the Mutare Naturam, select one pet, and then type in the name of your other pet you want to swap names with.
Using the Mutare Naturam is easy and is just like the Create-A-Pet page, minus the ability to choose a color. If you change your mind, all you need to do is go back to your inventory or visit any other part of the site. You will still have your Mutare Naturam. It won't be used/consumed until you actually make your choice so feel free to take as much time as you need!
This mystical tome will grant you the knowledge to look after an additional pet!
Using this grimoire will grant you one extra pet slot. As of now, you can have an unlimited number of pets on your account so there's no limit to have many Animalis you can use!
The Animalis stocks extremely rarely in the Magical Bookshop for 75,000 NP.
It can also stock in the Hidden Tower for 200,000 NP.
It generally sells for around 200-300k in users' shops.
It is possible to get an Animalis very rarely from dailies as well as Random Events. There is one infamous Random Event in particular where a Travelling Library holds a raffle for grimoires. If you have 10,000 NP on hand, you will receive either an Eritique Arcus or an Animalis. Always keep at least 10K NP out at all times. You don't want to miss this RE!
This disorganised grimoire seems to be jam packed full of pieces of maps.
Using this grimoire will grant you three random treasure map pieces except for Forgotten Shore map pieces. This could change in the future once more Krawk Island features get implemented.
The Occultus Loco stocks extremely rarely in the Neovian Printing Press for 50,000 NP.
It can also stock in the Hidden Tower for 150,000 NP.
It generally sells for around 200-400K NP in users' shops.
It is possible to get an Occultus Loco very rarely from dailies as well as Random Events. There is one Random Event that awards you a random treasure map piece and the set it belongs to varies with the day. If it doesn't give you a piece of the Secret Laboratory Map, there is a 1/10 chance that it will give you an Occultus Loco instead of a map piece.
It seems like whereever you put this book, it magically fits into the location with ease. I wonder if you could learn something from this...
Using this grimoire will grant you 25 additional slots for your inventory, shop, or gallery. You can only choose one.
The Spatium Procuratio stocks extremely rarely at Brightvale Books for 30,000 NP.
It can also stock in the Hidden Tower for 100,000 NP.
It generally sells for around 30-70K NP in users' shops.
It is possible to get a Spatium Procuratio very rarely from dailies as well as Random Events. There is one Random Event where the Shop Wizard Jubjub shows up and gives you the grimoire.
Notes on usage
Using the Spatium Procuratio and choosing shop/gallery slots will actually upgrade the size of your shop/gallery. For example, if you gallery is size 47 and upgrading costs 9,400 NP, if you use the Spatium Procuratio to get 25 extra slots, upgrading your gallery will then start costing 10,400 NP. So it's best to hold off on using this grimoire until the cost of upgrading five times is more than the cost of a Spatium Procuratio.
Also, this grimoire isn't used/consumed until you make your choice. If you use it and then change your mind, you can just go back to your inventory or visit another part of the site and you will still have it.
Infused with billions of colours, this book seems to radiate prismatic energy, perhaps the magic inside could be redirected to another location and used...
Using this grimoire grants you one free dip in the Rainbow Fountain.
The Eritique Arcus stocks extremely rarely at the Faerieland Bookshop for 150,000 NP.
It can also stock in the Hidden Tower for 1,500,000 NP.
It generally sells for around 800-999K NP in users' shops. The price will most likely increase later on in the year and drop again during giveaways.
It is possible to get an Eritique Arcus very rarely from dailies as well as Random Events. There is one infamous Random Event in particular where a Travelling Library holds a raffle for grimoires. If you have 10,000 NP on hand, you will receive either an Eritique Arcus or an Animalis. Always keep at least 10K NP out at all times. You don't want to miss this RE!
Notes on usage
Using this grimoire immediately uses/consumes it and you will get one 'slot' for a dip in the Rainbow Fountain. You can't undo this but you don't need to paint your pet immediately. You can wait until you're ready.
Also, make sure you go to the Rainbow Fountain in Faerieland and not the Rainbow Pool in Neopia central. Using the Arcus is basically the equivalent of completing a (very expensive) Fountain Faerie Quest on retail Neopets, after all. You can even get the Buzzin avatar this way.