By integrating Books - CRM - Projects, we realized that Customer are imported from the CRM into Books and they have at this stage the expected currency is associated with the customer (say USD). Then when Project is integrated, something strange happens: Projects and timesheets are pushed with their own currency and if different from the original customer one (say EUR), it is changed by the system (then to EUR). When you want to change it back to USD, the system tells you it is impossible because "transactions are existing". After analysis we realised that these "transactions" are not invoices or even sales order, it is because of the Timesheets linked to projects that were pushed into EUR!!! This is crazy because, it is not because a project is budgeted in EUR that I do not have to invoice the customer into its original currency (here USD). But then it gets better:
The only way to get the currency back to USD is to erase all the Projects and all the Timesheets for this customer, not into Books (that would be too easy ;-) but into Projects (because even if you break the integration with Projects, Books refuses to erase the Projects and Timesheets...). So, how can I explain my project manager, he will have to erase tens of projects and hundreds of timesheets to recreate them again afterward!? Also, even changing the currency of the project and timesheets back to USD in Projects does not help, because when synchronizing, Books only accepts the currency "blocked" for this customer, this means EUR!!! (chicken and egg problem).
All in all, my suggestions are:
1) to be able for a customer to change the currency at ANY TIME because it can happen that a customer that was once in the past invoiced in USD needs to be invoiced in EUR in the future (they have the currency for any reasons on their bank account, accounting can very well cope with that). And this should not neither be blocked by or change the past transactions (existing invoices or sales orders). Eventually, you can do this with a "currency change" flag to be activated in Customer settings in Books and a warning when a currency is changed that all future transactions will happen in the new currency.
2) It should be possible to delete any Projects related input (projects and timesheets) coming from Zoho Projects once the integration is broken. Eventually the deleted "transactions" (wrong name for projects and timesheets actually as they are not really transactions) will reappear in Books once the integration with Projects is re-established (which is logical).

Conclusion: Until currency can be modified for our customers as indicated in the CRM, Books is totally useless for us as the rest of the Zoho Finance suite.