At the moment, there is no option to reimburse expenses from Zoho Books. The option will be available soon.
As of now, You can mark your expenses as reimbursed only from Zoho Expense. When a reimbursement is recorded in Zoho Expense, transactions will be automatically posted in Zoho Books. You can view those transactions under the reports section of Zoho Books (Reports > Accountant > Account Transactions > Account name: Employee Reimbursement).
Note: As of now, Zoho Expense only supports offline reimbursement.