Air carrier First Officer reported a taxiway incursion and signage confusion regarding taxi instructions given by ATC. The crew was taxiing for Runway 15R in IAH using the West 2 standard route but hold-short NE; a route where taxiway NE changes to taxiway WB.

Narrative

We were on day three of a very long four-day trip with relatively short overnights. We closed the main cabin door early in IAH (IAH-ZZZ) but had a long delay before pushing back because of ramp congestion. We finally pushed and were cleared to Spot 5 and contact Ground Control. Upon contacting Ground; we were instructed to taxi to Runway 15R via the west 2 route but hold-short NE. I had looked at the West 2 but needed to review it to find NE. I didn't see NE depicted on the 'Standard' route chart; so I switched to the airport diagram. I also saw signage outside to the left that seemed to indicate that we were taxiing through WB; and I misread the chart to read that NE was an E-W taxiway further and to our right. What I thought was taxiway NE was actually NB; not NE. In fact; we were taxiing through NE; which apparently changes to WB near our intersection. I had originally thought NE was WB since I saw the WB signage to the left and 'WB' on the airport diagram to the south on the same taxiway. I didn't realize that WB changed to NE either at; or slightly before our intersection. Later in the taxi process; we were told 'Company (no number) contact Tower 127.3'. Since there was no number as part of the clearance; both the Captain and I missed the switch over. Later I realized that; despite the company page emphasizing in the first remark specifically to NOT switch until told to do so by ATC; the West 2 states to make the switch without further instruction. This contradiction; coupled with the lack of flight number from Ground; added to the overall confusion.More review of the standard taxi routes would have been helpful. We briefed potential taxi routes; but including the Standard Routes; there were multiple possible routes. The changeover from taxiway WB added to the confusion; as did our clearance to taxi to Runway 15R with essentially an immediate hold-short instruction. The chart depiction (not AMM Airport Moving Map) seems to indicate WB to our left; and NE possibly E-W orientation further on in the taxi route. The AMM would've been better; but only after zooming in quite a bit. Even then; NE appears to be WB. This is a threat area. Finally; there is a contradiction between the company page (do NOT switch until specifically instructed to do so) and the Standard Taxi route instruction to switch without instruction. We also were instructed to switch; but without our actual flight number.