I would suggest the MOON Nicaragua guidebook.
You may find cheaper to fly to San Jose (SJO) Costa Rica with so many flights and usually the lowest prices of any capital in CR, at least from the US. Iberia via Madrid, and Condor via Germany? There direct flights in US via the following cities to all capitals in CA, and also, there is a newer airport closer to Nica border in CR Liberia (LIR), just 2 hours bus ride to border.
American is via Miami (MIA), United is direct from Chicago now (ORD), and I think from Newark New Jersey (EWR), and check TACA from JFK NYC.
Delta/KLM from Atlanta (ATL)
American from Dallas (DFW), and United from Houston (IAH).
Taca also serves all of CA capitals, though not all direct from each capital to another,
The bus from San Jose CR to Managua is direct on King Quality or Tica Bus.
You can sometimes catch the same bus in Liberia, or there are direct busses to border every 30 minutes. From border, you can taxi hire to San Juan Del Sur or to San Jorge via Rivas for Ometepe. Or just wait for the busses to Rivas and switch there.